Typically the assumed dynamic in heterosexual relationships is that men are masculine, aggressive, dominant and women are feminine, passive, submissive. In sexualities where the partners are the same gender, the exact same traits apply. People who have "masculine" traits are typically attracted to "feminine" traits, and vice versa, the only difference is whether the object of your affection is a man or woman. Knowing that, it's upsetting to see that more people don't understand how similar we all are.
In the cases of homosexuals of both genders, the sub-orientation groupings of masculine/feminine, "top"/"bottom" in gay men, likely emerged in order to help individuals define the way they see themselves and who they are attracted to. If these individuals fell into the trends described by the categorization, it is logical that the recognition and vocabulary to define these trends would follow.
The stereotypes of heterosexuals are already well defined, which is perhaps why there isn't as strong of a trend in sub-categorizing sexual orientations in heterosexual culture. Women are assumed feminine and men are assumed masculine; there is little perceived variation within each of these gender/sexuality combinations. Of course not all people follow these trends, but many people do, or at least feel that they are supposed to. It's less acceptable in our society as a whole for a woman to be dominant/masculine, or a man to be submissive/feminine, which is why there is less open discussion of classification of these groups.
Homosexuals have already kind of shrugged off what society as a whole deems to be acceptable, which may be why there is more open discussion and classification of the way that gay culture operates. Indeed, the homosexual individuals that I talked to seemed much more comfortable talking openly not only about the group dynamic of their sexuality, but their own experience and preference as well.
The sub-orientation of "tit man" vs. "ass man" that started this whole project is a little bit frivolous compared to what I learned about the inner workings of homosexual culture. The practical meaning of that distinction is virtually non-existant in dealings with women; most women that I spoke to about the issue either thought it was funny, didn't care, or thought it was offensive. Men don't usually talk to women about it, and if they do the women don't really care. In comparison, the sub-orientations in gay culture are very important in determining what type of person you end up being attracted to. The "orientation" of "tit man" or "ass man" turns out to not be very important when dealing with relationships. It's more of a somewhat meaningless argument that men usually have exclusively with each other.
On the whole, I kind of feel good from learning this. It's a relief to see some type of sociological grouping emerge from a group, for a group. "Tops" and "bottoms" aren't terms invented by doctors as they dissect homosexuals trying to find out why they're inferior, which is sometimes the case with the invention of terminology to describe groups of people in history. They're simply terms invented by members of the gay community to facilitate communication about their interests and desires.