Now, we're not saying that these theories behind our sexual behaviors are the gospel truth or that there aren't other, conflicting theories out there. But if they are true, sex is even weirder than we thought.

Ever since Darwin popped off his big theory of evolution, we've all kind of presumed that everything we do -- the way we eat, the mates we choose, the way we purposely mispronounce "indubitably" for laughs, everything -- is ultimately tied to one goal: continuing the species. But sometimes getting from A to Baby isn't as intuitive as you'd think. And scientists have had a hell of a time figuring out why.

5 Kissing Evolved as Virus Protection

Long before you tasted the wonders of sex or the body parts that have to do with sex, you (hopefully) tasted the inside of another person's mouth (if not, you should probably get off Cracked and finish your pre-algebra homework, sonny). But have you ever stopped in the middle of a deep kiss and said, "Wait a second, why the hell are we doing this?"

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"OK, now I just consume your head whole, right?"

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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.

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The tattoo equivalent of mustache crabs.

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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.