The 5 Weirdest Reasons We Have Sex (According to Science)

Pretty much all human cultures have kissing, and a few other animals do it as well. But why? Why are we cramming our mouths together? Why don't we nuzzle noses or tap kneecaps? Why don't we butt rub?

The Theory:

Researchers at the University of Leeds have hypothesized that kissing evolved as a way for women to expose themselves to an infection called cytomegalovirus. Never heard of it? Maybe you've heard of its family -- herpesviruses (we're not missing a space between "herpes" and "viruses" -- that's the actual name of the family). The thing about this particular virus is that, much like a Looney Tunes tramp stamp, you don't know if your partner has it until it's too late. By then you've already exposed yourself to something you'll never shake.

And that's bad news for the species -- if a woman develops the active cytomegalovirus while pregnant, there's a 50 percent chance she won't carry the baby to term. But, if she gets exposed in incremental bits in the months leading up to her pregnancy, she can be inoculated from a full-on primary infection. Think of it this way: The virus is kind of like early 2000s boy band music. In small doses, it's not so bad. In large doses, it will abort your baby.

You can see where we're going with this -- the easiest way to transmit the virus is through swapping saliva. The theory is that, rather than moving straight into intercourse (which could immediately get a woman pregnant while giving her mega-herpes and maybe killing her unborn child), females created this courtship-slash-inoculation period -- the kissing season. Which is a way better and less gross theory than the first draft -- that kissing evolved as a natural result of mothers prechewing food for their young (and lovers).

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Stick this in your lip and chew it: Human breasts are 100 percent unique. If you were into animals in a sexual way, you wouldn't be able to find any with boobs unless they were lactating -- only human women don't go back to flat-chestedness when they're off milk duty. True, gravity and age will eventually take their toll, but barring disease or a tragic mammogram/waffle iron mix-up, a woman's breasts are there to stay.

So the question is why? Why were humans, more than any other mammal, bestowed the bosom blessing?

The Theory:

Boobs are designed so babies don't suffocate and die while nursing. Take a look at this face:

OK, now look at this totally not adorably pathetic photo of a baby monkey:

Notice the difference? Look closer. It's the mouths. Most mammals have protruding snouts that jut out like tobacco-dipping animal hillbillies. They were made that way. Humans, not so much. Try this little experiment: Staple a rubber nipple to a wall and try to suck it. It doesn't work, does it? Not just because you're suckling a wall, but because your nose is smooshed against a flat surface.

Hurry, back up, before you die! So the theory is that as humans evolved flatter faces to offset our huge brains during labor, women evolved bigger bosoms to reach their babies' little tiny mouths. Lucky for humanity that women hate it when their babies suffocate. That's literally the worst.

Related: 5 Amazing Things You Didn't Know Babies Could Do

Of course, the frequency and loudness and squeakiness of sex noises vary from culture to culture, but we do know that there are other primate females who also make noises during sex (we're looking at you, slutty bonobos). And we also know that at least some of the noises gushing from the woman's mouth have nothing to do with orgasms.

It seems like there's another reason here that we're missing. Authors Cacilda Jetha and Christopher Ryan think that it has to do with promiscuity.

The Theory:

In their 2010 book, Sex at Dawn, Jetha and Ryan put together a startling theory -- that humans aren't meant to be monogamous, that we were never meant to be monogamous and that biology itself is what's tearing modern marriages apart. And among the justifications for their theory, Jetha and Ryan cite female copulatory noises.

Remember how we mentioned female bonobos like to make lots of noises when having sex? The funny thing about this is that bonobos are just as related to humans as chimps, and they do a lot of the same stuff that we do sexually, like French kissing and oral sex and face-to-face intercourse.

They're also not monogamous, not by a long shot. And here's where it gets interesting: When female bonobos are screwing around, they make lots of noises, not because they're having a good time, but so the other males in the vicinity will hear them. It's a mating call. An invitation. An alert to the other boys in the area that it's party time and she's open for business.

Now, if bonobos are in the top two of our closest relatives, and they totally are, and they've never bought into this whole one man, one woman business, does that mean we should all try to Gingrich our marriages? Probably not. But you definitely should make sure the neighbors aren't around the next time you and your lady get it on.

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Compare that to the ways that some far stupider animals reproduce. For example, aphids don't need boys at all -- the girls just fart out clones of themselves every 10 minutes or so. Some starfish just have to shed a limb to make a baby. OK, maybe that's not the best example of a better way to do things. But it's definitely simpler.

So why didn't humanity take a different turn -- one where we kept the species going by just cloning ourselves? Why can't we and other sex-having species just squirt out spores from our orifices and call it a day?

The Theory:

The answer is that sex -- the mingling-of-fluid kind -- results in constant adaptation. Every kid is carrying the best of her mom, her dad, her grandparents and their lovers. Every one of us is a mess of genetic material coming from all over the place. And that's good, because change is what keeps us ahead of the game when it comes to our biological enemies.

According to the Red Queen hypothesis, we have to keep adapting to keep moving forward. The theory comes from a scene in Through the Looking Glass when Alice and the Red Queen race but never move. The queen says, "It takes all the running you can do, to keep in the same place."

Likewise, humans are in a kind of arms race with every other species, especially the ones that matter to us most: parasites. Animals who reproduce asexually never get a chance to mix things up -- to come up with new and better combinations of genes for the next generation. That makes them more vulnerable when a parasite comes along.

Scientists have even proven the Red Queen theory in the lab. No, not by hosting a couple and watching them breed, then comparing their medical weaknesses to a family of human clones. They just used a particular bacteria and its viral parasite. The nice thing about bacteria is that you can watch hundreds of generations evolve over a short amount of time. So they took one bit of bacteria and isolated it from its parasite partner. Then they took another bit of the same kind of bacteria and let it co-evolve with its parasite, just as it would in the wild.

Five minutes and thousands of generations later, the second group had evolved twice as fast as the first, with more mutations and diversity. Then, just to be mean, they took the virus from the second group and infected the isolated bacteria with it. The bacteria was annihilated. If we didn't have sex, that bacteria would be us every time we came across a new cold.

Related: 6 Horrifying Parasites (That Could Be Inside You Right Now)

The male orgasm might be a little messy, but at least it makes sense. Everything about the process of ejaculation is geared toward either human-making or keeping the tissue industrial complex going. God knows we need both if we want to continue as a species. But the female orgasm is a different (cleaner/sexier) animal. The only thing anyone gets out of a female orgasm is a good time.

So, as a quick review, there are two kinds of lady orgasms. The rare kind is vaginal, the regular kind is clitoral. When you hear the term "G-spot," they're talking about vaginal orgasms (on the inside). Between 70 and 80 percent of people of the female persuasion get their orgasms via clitoris (on the outside).

So experts have proposed all kinds of theories to answer the question of why women have orgasms at all. Like that the contractions are supposed to suck sperm toward the uterus and that orgasms are supposed to exhaust women so they'll stay horizontal, so gravity will help sperm hit its mark. But one theory stands head and more head above the rest.

The Theory:

The clitoris is the remains of what would have been a penis. You might even say it's a pre-penis.

According to biology professor Dr. Elisabeth A. Lloyd, there is zero evolutionary purpose for the female orgasm. Zero. It does absolutely nothing for procreation, nothing to advance the species. But the reason women have them is the same reason men have nipples -- leftovers. Here's the idea: We all start out as sexless tadpoles in the womb. Before the boys become boys and the girls become girls, the basement starts developing nerve and tissue pathways in the exact same places. Then God gives us our assignments and we start our path to Gendertown. But that order -- nerves first, genitals second -- is the reason women get to share in the goodness of orgasms. And maybe why the clitoris looks like it does.

In other words, gentlemen, the lady you're boinking is sporting a baby penis, despite her constant protestations to the contrary.

Kristi Harrison is blushing her head off at Twitter and Tumblr right now. You can follow her if you want.

For more on sex that'll turn you away forever, check out 9 Awesome Places to Have Sex (And the Horrific Consequences) and The 10 Most Terrifying Guides to Sex.

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Check out Robert Evans' A Brief History of Vice: How Bad Behavior Built Civilization, a celebration of the brave, drunken pioneers who built our civilization one seemingly bad decision at a time.