Libidoism, or: Why we define gender based on sex drive

Guys do it, girls do it and, it turns out, even biologists do it. Take the fruit fly, for example: Evolutionary biology says that the reason that the fruit fly's sperm are so small and the eggs are so big is directly related to each sex's view on sex. The female fruit fly is programmed to value reproduction highly and be invested in picking the right mate, where the male fruit fly just wants to get it off as much as he can. Hence, smaller sperm. This theory can be applied to a vast number of species including, ahem, humans. But, how does that explain an experiment where female and male fruit flies were shown to be similarly choosy when deciding with whom to get it on?
Girls are brought up, from the moment we are taught about boys and their boners, to believe that guys are mostly indiscriminate in their search for booty. And by the same token, men are taught to believe that sex will always be emotional for women, and that they will have to jack off for days once married because she'll be too stressed/tired/emotional/frustrated/angry to want to do it that night. And haven't you ever heard the "boys will be boys" "men can't help but cheat" crap? Yes, crap. Its always bothered me, this double standard that expects men to be having sex as often as they can while women wait, or are at least much more choosy and emotional about their sexual partners. My question, and I'm sure every other woman's question when first introduced to that idea, was simple: "Who, exactly, do you think these guys are supposed to have sex with, then?"
So, women have a double standard despite three waves of feminism and a sexual revolution. The really annoying thing, though, is that sometimes we put this standard on ourselves. Even those of us who are "sexually active" and not yet married/engaged thus defying traditional standards seem to have certain expectations of what our and our fellow female's sex drives should be. We are all expected to laugh at and understand the old "He wants it all the time, I'm fucking tired!" joke and be able to commiserate about our unfailingly horny husbands/boyfriends/"friends"/partners. But, what about those of us who aren't? What about those of us who find ourselves in that person's bed one morning faced with a seemingly astounding realization: that we, in fact, have higher sex drives than our partners. What does this mean for our femininity? And, more so, for his masculinity? I have been guilty of complaining about my ex boyfriend's lower sex drive, using the phrase, "It's like I'm dating a girl!"And hearing back, "Are you really, really sure he's not gay?" If a man is too tired/stressed/emotional/frustrated/angry to have sex, why do we suddenly assume that something must be wrong with him when we expect ourselves to follow that standard? If a woman wants to have a lot of sex that doesn't necessarily (or too often) need to involved emotion to be satisfied why do we think she has some underlying problem?
If we take a step back and remember the fruit flies, we'll see that they, too defy these gender standards, so why can't we? Why do we feel the need to generalize something that is so complex? I know that I have felt less feminine as a result of my high sex drive, and felt that maybe something was wrong, and I assume that there have been men in the past who felt less masculine as a result of their emotional connection with/lower need for sex. So, if we know these exceptions to the rule exist both socially and scientifically, why do we continue to define the genders this way?
I don't know, but I do know that I'd like to tell that ex boyfriend that I'm sorry. I'm sorry I called you a girl, because the truth is I thought it was impossible for you just not to want it and felt offended when you wanted to sleep. I took it as a reflection of my attractive...ness... in your eyes, and thought that if you, a man, didn't try to sleep with me every time we were in the same space you must not want me. I'm sorry I held you to a standard that didn't reflect who you were at all, and I'm sorry I gossiped instead of telling you that I wasn't satisfied. I will try in the future to judge a man by who he is, not by how well he matches up to the socially constructed masculine identity, in hopes that he will not see me as less of a woman for wanting more (less emotional) sex. And, you know, my insufferable "guy nod" habit. :)