Here are a bunch of "Sex is Like..." metaphors that I found from random websites...
Sex is like snow; you never know how many inches you’re going to get or how long it will last.
Sex is like your salary; you don’t disclose what you get but you always think that others get more than you.
Sex is like playing bridge; if you don’t have a good partner, you’d better have a good hand.
Sex is like money; only too much is enough.
Sex is like air; it’s not important unless you aren’t getting any.
Sex is like algebra to some; they understand the fundamentals but simply aren’t interested in it.
Claiming that sex education leads to irresponsible sex is like claiming that driver education leads to car accidents.
Sex is like math; add the bed, subtract the clothes, divide the legs and pray to god you don’t multiply.
Sex is like a knife; You can use it to make something wonderful or you can use it to wound someone, it depends on the person wielding it.
Sex is like a Chinese dinner; It isn’t over ‘til you both get your cookie.
Sex is like spinning a quarter on a counter; Even if you give it a really strong start, it will eventually wind down if you don’t do things to keep it spinning.
Sex is like art; Most of it is pretty bad and the good stuff is out of your price range.
Sex is like bowling; Be spared from striking out.
Sex is like voting; When you come of age, you get to exercise your right.
Sex is like chocolate; When you meet somebody who doesn’t like it, you go “what?! No, you must like it! Everybody likes it!”
Sex is like politics; The more you learn and read about it, the better.