[My friend nextworldover wrote this as a Reddit comment in response to a person who grew up conservative Christian and did not know anything about sex. I thought it was an excellent explanation of how sex works and asked to post it as a guest post.]

People have sex for many reasons, the most common of which are pleasure, bonding, and reproduction. Stimulating erogenous zones (including genitals but also other sensitive areas on the body) feels pleasurable to most people, and can relieve sexual arousal. Sex is often an intimate and vulnerable experience, which many people find deepens their emotional bond with their romantic partner. Sex is also the way that we conceive children.

Pleasure: When aroused, the genitals generally become engorged. For a penis, this is visible as an erection. For a vulva, this is visible as the labia and clitoris swelling and appearing flushed. Arousal also frequently comes with secretion of lubricating fluids. For a vulva, this fluid will originate near the entrance of the vaginal canal, within the labia minora. For a penis, it will secrete from the urethra. Here‘s a diagram of how the penis and vulva develop from the same structures. This might help you get some idea of how the anatomy fits together.

People have differing amounts of desire for sex and differing patterns of arousal. Some people have high libidos, and frequently find themselves desiring sex. Some have low libidos, and only want sex occasionally. Some people have none, and never desire sex. Some people have spontaneous arousal, and find themselves easily aroused to sexual desire regardless of context. Some people have responsive arousal, and only become aroused once they are in a conducive context. Libido can be affected by age, sex, health, life stress, medication, mental health, and more.

There are many things people can do together to cause each other sexual pleasure. Some popular ones include: masturbation, fondling, making out, manual sex (handjobs, fingering), oral sex (blowjobs, eating out), penis-in-vagina sex, and anal sex. Many people find that there are scenarios, activities, body parts, or objects that assist in their sexual arousal or pleasure. Most of these can be explored one one’s own or with a willing partner. A common sexual aid is pornography, which can provide mental stimulation. It can come in many forms, including writing, drawings, games, photos, audio, and videos. Many people enjoy consuming pornography by themselves or with partners as part of their sex lives. Another common sexual aid is vibrators, which can provide physical stimulation. Vibrators can be applied to any erogenous zone, including but not limited to the nipples, clitoris, vagina, penis, anus, and prostate, and often they are designed to be more effective for use with a particular one. Many people enjoy using vibrators on themselves or each other as part of their sex lives. There are many, many, many forms of sexual aids, catering to a wide array of tastes.

Here is a diagram of common human erogenous zones. Many people like to touch these areas on their body, or stimulate them with objects, or have other people touch them. Due to differing physiology and psychology, every person has a different set of preferences about how they prefer to be touched. Some places that feel good to some people will feel uninteresting or unpleasant to others. Likewise, some sorts of touch that feel good to some people will feel uninteresting or unpleasant to others. The best way to discover what you prefer is to experiment with different things. The best way to discover what someone else prefers is to ask them. However, communicating this sort of thing can be difficult, people may not know what they want, and executing a particular action in the desired way may require practice. For this reason, the experience of sex usually improves as you gain more familiarity with your own preferences and those of your partner.

Bonding: Many people find that when they have sex with someone, they feel more emotionally connected to that person. Having sex is a very intimate experience during which people often feel very vulnerable. Having sex in a way they feel comfortable with in a safe environment with a person they trust is considered by many one of the most relationship-affirming activities they do. Many people have insecurities about their appearance, performance, or general desirability as a sexual partner. During sex is a time when acceptance or rejection is likely to hit someone especially hard. Some people require being in an intimate relationship in order to enjoy sex at all. Some people enjoy having sex outside of intimate relationships. Sex does not always engender feelings of closeness with the other person. Some people find that sex outside of a relationship does not provoke much emotional response in them, or that no sex does. Sex can also be frustrating, upsetting, or traumatic. Feeling unable to communicate what you want, or feeling that it cannot be achieved, can be frustrating. Feeling rejected by the other person, or like they don’t care about what you want, can be upsetting. Sex with someone you don’t trust or feel safe with, or with someone you trust but who abuses that trust, can be traumatic. Sexual trauma often arises when people have nonconsensual sexual experiences – this often involves feeling violated and ashamed. The best way to avoid causing sexual trauma to yourself or your partner is to progress slowly and check in often about what you’re each comfortable with, and to care for each other’s needs, wants, and boundaries.

Someone’s sexual orientation refers to which people they are attracted to. Someone who is heterosexual finds people of the other gender attractive, someone who is homosexual finds people of their own gender attractive, someone who is bisexual is attracted to people of more than one gender, and someone who is asexual is not sexually attracted to anyone. These are terms people use to try to describe their experiences, and many people find that these descriptors might not perfectly describe their sexuality. For example, someone might be mostly attracted to men, but also much less frequently attracted to women. Someone might be attracted only to people with breasts, regardless of their gender. Someone might like engaging in some sex acts with women, and others with men. Someone might find that they are capable of physically enjoying sex with any gender, but that it only gives them an emotionally satisfying experience with one. Sexuality is rarely completely clear cut, and the words someone uses as a shorthand for themself are just that.

Reproduction: Sex is the most common way that people conceive children. Sperm are produced in the testicles and are ejaculated from the urethra of the penis along with seminal fluid. When introduced to the vaginal canal, sperm can join with an egg and fertilize it. This can also be performed via artificial insemination. Eggs are released from the ovaries at a rate of approximately one per month according to a hormonal cycle. This hormonal cycle also regulates the buildup of endometrium in the uterus. The egg travels from the fallopian tube to the uterus, and if not fertilized, will pass out of the uterus with the endometrium as part of menstruation. Menstrual fluid usually passes out of the body over the course of around 5 days, during which time people generally use one of a variety of hygiene products to absorb or collect and discard it. Some of the more popular options include pads, tampons, and menstrual cups. Menstruation can involve the uterus cramping as it expels the fluid, as well as a variety of other symptoms such as headache, diarrhea, bloating, irritability, sensitivity, increased or decreased libido, breast tenderness, fatigue, and more.

In vitro fertilization involves fertilizing the egg outside of the body, and then placing it into the uterus to implant. If the egg is fertilized, it will implant into the endometrium as an embryo and develop into a fetus. Pregnancy in humans lasts around 40 weeks and is generally divided into three trimesters. Here is a chart of fetal development over the course of pregnancy. With current medical technology, after about 32 weeks of development, babies are generally capable of surviving outside the womb, though they may have some health complications. Babies can be born vaginally, in which case the opening of the cervix must dilate large enough for the baby to pass through the vaginal canal. This is generally considered to be an extremely painful experience for the person giving birth. They can also be born via C-section, which involves performing abdominal surgery to cut open the uterus and extract the baby directly. Because it is a fairly major surgery, it can also be extremely painful, and require time for the incision to heal. After birth, the cervix and vaginal canal may undergo some changes – these may be minor changes in shape, or may involve more major injuries, such as tearing. For most people, the vaginal muscles regain their tone within weeks of birth.

For those who wish to avoid pregnancy, there are many options of varying ease of use, efficacy, and side effects. Here is a chart that covers many of the more common methods.

Condoms are one of the most popular and effective forms of birth control. A condom is a very thin latex sheath which covers the penis, catching semen and preventing sperm from entering the vagina. Condoms also protect against a variety of sexually transmissible infections. Condoms are convenient because they are widely available, inexpensive, quick to apply, and highly effective when used properly. The drawbacks of condoms include reduced sensitivity and variance in people’s ability to reliably use them correctly. A more reliable and permanent form of birth control is the vasectomy, which is a minimally invasive procedure that involves severing the vas deferens, the tube through which sperm travel from the testicles to the penis. Benefits include that once the procedure has been performed, it will remain effective until reversed. It is frequently although not always reversible. Drawbacks include pain or discomfort during the procedure and potential permanency.

For those with ovaries, a highly popular form of birth control is birth control pills. These pills, when taken correctly, alter the hormonal cycle to make hormone levels more consistent, which prevents ovulation from occurring. They also frequently help with the various symptoms of menstruation, making them popular even among people who are not sexually active. Different formulations of the pill exist, and have somewhat different side effect profiles. The pill can generally be obtained by prescription after discussing it with your doctor. Depending on various factors, such as whether it is covered by insurance, it can either be very inexpensive or quite expensive. The pill is convenient because it does not require use during sexual activity and is highly effective. Drawbacks include that it must be taken daily to be effective, and that it can cause a variety of hormone related symptoms, such as those listed above for menstruation, and others, such as increased risk of blood clots and stroke. A third highly popular form of birth control is the intrauterine device, which is a small piece of plastic which a doctor can insert into the uterus. There are both hormonal and nonhormonal versions, with different side effect profiles. It is convenient because it is long-lasting, reversible, extremely difficult to use incorrectly, highly effective, and, in the case of the hormonal IUD, it can alleviate menstrual symptoms or prevent menstruation entirely. Drawbacks include the same ones as hormonal birth control, if it is the hormonal IUD, as well as pain during insertion, heavier menstrual flow, and more painful menstrual cramps. There are many other forms of birth control available, including the shot, the ring, the implant, and more. Each comes with its own benefits and drawbacks.

Although this is a long post, it is only a cursory introduction to the topic of sex. If you are curious to learn more, there are many online resources:
Scarleteen is a popular, well-maintained, and informative sex ed website.
Oh Joy Sex Toy is an online comic that reviews sex toys and produces sex ed materials.
Archive of Our Own is an online repository of amateur fiction. A lot of it is erotic in nature.
PornHub is a popular site for video porn.
The Guttmacher Institute publishes research on topics of sex and sexuality.