The Biology of Sex

Sex is not just something many of us like to do whenever we have the time and inclination. The act is the sole reason we're all here. The vast majority of animals (but not all) have sex to procreate.

Let's explore this reproductive phenomenon, sometimes referred to as intercourse, coitus or copulation.

What is sex?

"Sex is the transfer of reproductive material from one organism to another in order to create offspring," said Danae Maragouthakis, M.P.H., the medical director at Yoxly, a company based in Oxford, England, which creates a range of remote sexual health services, including STI test kits.

Maragouthakis referred to "sex" in terms of biology and the way the word is used by biologists and scientists to describe how the human species reproduce and create children. That's very different from how our modern society instructs language, and the term "sex" in everyday conversation is no longer used solely to describe a method of reproduction, but instead describes how a huge diversity of people express and experience sexual pleasure.

In biological terms, humans have penetrative sex when the penis, a reproductive organ, enters the vagina, a reproductive tract, and releases sperm that travels along this tract to meet and fertilize an egg, creating a new organism, she explained.

"This fusion of gametes—that is, the egg and sperm—recombining parental genomes into a new genotype are what we call 'sexual reproduction,'" she added.

Why do humans have sex?

The answer to the question of why people have sex might seem obvious, but as with anything in humanity, it's actually quite nuanced. Maragouthakis confirmed that in terms of traditional biology and evolution, the primary purpose of sex is reproduction.

"Sex quickly creates greater genetic diversity, thus, conveying evolutionary advantages that result in healthier offspring," she said.

However, she pointed out that relatively recent medical technology advancements make it possible to reproduce without sex, using the systems of in vitro fertilization (IVF), intrauterine insemination (IUI) and intracytoplasmic sperm injection (ICSI).

There are also a number of ways to ensure that sex does not result in reproduction, in the form of contraceptive options such as barrier methods (condom, diaphragm), hormones (birth control pill), intrauterine devices (IUDs) and surgical procedures like vasectomies and sterilization.

Maragouthakis listed much more complex reasons—by category—that we have sex beyond basic biological purposes:

• Pleasure (physical)
• Relaxation (physical)
• Power (psychological)
• Insecurity (psychological)
• Bonding (emotional)
• Love (emotional)
• Resources (social)
• Pressure (social)
• Duty (cultural)
• Religion (cultural)

Societal attitudes toward sex

While the way we have sex has remained the same for millennia, the way we perceive sex is constantly evolving.

"Topics—where people have varied perspectives and attitudes—include gender fluidity, transactional sex, erotica material, how sexual needs and desires are expressed, and the use of sex toys in relationships," said Katherine M. Hertlein, Ph.D., L.M.F.T., a professor in the couple and family therapy program at the University of Nevada, Las Vegas.

On the political spectrum, these opinions can range from liberal to conservative. These feelings can be formed by previous experiences, family influence, society and cultural mandates, she explained.

"While the reins are loosening, there are still many that hold on to a more traditional view of sex—that is, sex with one partner, sex in a relationship or marriage, sex is private—while others are much more open to diversity with sex," said Debra Laino, a sex therapist and educator in Delaware who is certified by the American Association of Sexuality Educators, Counselors and Therapists (AASECT).

She described America's attitudes toward sex as "split," something she attributes to the omnipresence of sex without adequate sex education or healthy conversations to contextualize it. However, Laino pointed out that premarital sex, cohabitation and same-sex relationships are more accepted today because people are exposed to and educated about them much more, even if only informally. She surmised that access to technology means younger people are becoming more accepting of diversity within sex.

Myths and misconceptions about sex

Sex education and discussions about sex can be taboo, lacking or nonexistent in places, so myths and misconceptions about intercourse and sex in general abound.

"Probably one of the biggest misconceptions about sex is that real sex is like a porn video, which causes a tremendous amount of pressure to look and sound a certain way," Laino said.

She added that there are some truly strange and dangerous myths out there, especially among teenagers. For example, here are some of the strangest:

• You can't get pregnant in a hot tub.
• If you drink a cap full of bleach, you will not get HIV.
• You should put yellow Skittles in your vagina after unprotected sex to stop any infection.

Laino was very quick to point out that all of these suggestions are false.

While there are too many myths to discuss in one article, Monica Grover, D.O., an OB-GYN and chief medical officer at VSPOT in New York City, detailed eight of the most pervasive:

Myth: All women orgasm during vaginal sex.

Reality: A study published in the Journal of Sex & Marital Therapy found 81 percent of women don't orgasm through penetration alone and require additional stimulation.

Myth: Condoms make sex less enjoyable.

Reality: Condoms reduce the risk of contracting a sexually transmitted infection (STI). An STI can mean having to avoid sex altogether until the infection has cleared or is no longer transmittable. That's less enjoyable.

Myth: If you're aroused, you shouldn't need lubricant.

Reality: Whether you're excited or not, lubricant can always make sex more comfortable and fun, as well as reduce pain.

Myth: You can't get pregnant using the withdrawal method, that is, withdrawing the penis before ejaculation.

Reality: About 1 in 5 people per year get pregnant while using the withdrawal method, also known as pulling out.

Myth: You can't get pregnant while on your period.

Reality: It is possible to get pregnant because menstrual cycles and ovulation can be irregular.

Myth: Oral and anal sex both fall into the category of "safer sex."

Reality: It's possible to get STIs from both oral and anal sex, for example, chlamydia in the throat.

Myth: You can tell if someone has an STI.

Reality: The United States has the highest prevalence of STIs in the developed world, but many can be asymptomatic. A study indicated that about 45 percent and 77 percent of all cases of gonorrhea and chlamydia, respectively, never display symptoms. It's not only that you can't tell if a potential partner has an STI, but that person may have no idea they're infected, either.

Myth: Plan B can be used as regular birth control.

Reality: The hormones in Plan B can disrupt periods, which can actually make it harder to avoid pregnancy.

Some fun facts and stats about sexual biology

Naturally, what people consider sex differs depending on factors such as whether an orgasm was reached, a person was giving or receiving stimulation, or a condom was used. A Kinsey Institute study published in 2010 indicated that 45 percent of contributors considered masturbation as sex, 71 percent thought oral sex was sex, and 80 percent categorized anal sex as sex.

The average age of losing virginity for males in the United States is 16.8 years and 17.2 years for females.

A 2021 survey sponsored by Adam & Eve, a sex toy company, reported that 21 percent of respondents had sex once or twice a week, 14 percent once or twice a month, 5 percent once a day and 19 percent not at all.

When couples are sexually dissatisfied, there is a greater risk of breakup or divorce, so an open dialogue about intimate matters could potentially save your relationship.

It isn't just humans that have sex for fun. Bonobos, our primate cousins, do as well, using sex as a social tool to manage relationships, among other purposes.

Whatever you're into, thinking about sex in different ways could open up new areas, whether it's something you want to try or gaining a greater appreciation or acceptance for the diversity of sexual needs that exist in the world beyond your own bedroom.