A sociobiological question (I guess)

What, if any, is the sociobiological reason for people who have more common preferences in sex partners to try to ensure that other members of the same sex follow the same preferences? Aren't they just creating more competition for themselves that way? Or is the whole thing just cultural?

Those of you whose preferences follow the usual trends have probably never noticed this phenomenon, but I found it very common both in Russia and in the US (it usually happens to young girls a lot more than to adult women, so I don't have sufficient experience with it in Finland, but I have seen it here too). A girl expresses a preference that is somehow sociobiologically unusual (say, prefers short men to tall ones) and immediately a chorus of older female relatives and peers tell her why she shouldn't. I've seen this applied to boys by their male relatives and peers, too, but don't know enough about it to discuss.

In my experience the kind of pressure is quite common, quite strong and totally futile. It is usually expressed a lot more strongly than other kinds of disapproval on one's taste (with one exception that will be mentioned below). For example, in the US (and probably elsewhere) white women relatively rarely have sex with Asian men, at least partially for sociobiological reasons (Asian men are shorter, etc.) I happen to like East Asian men well enough, and ran into an amazing number of white women who highly disapprove of the fact. For a comparison: I have also had sex with black men, and met women who disapproved of that, but they say things that imply that black men are trouble. The women who disapprove of my interest tend to say that either a) I am perverted or b) Asian men have small dicks and are bad in bed, although of course these women have no personal experience of their dicks or sexual skills. I've heard the same kind of "perversion" talks directed at me for my acceptance of short men (I prefer them average-sized but neither short nor tall bothers me much); I even got a few "you can't possibly want anyone that short" talks concerning a man who was in fact 178 or 179cm tall. Dating a younger man also elicits that kind of response in a lot of people. Ditto for dating a man who would not make the first move.

The only other exception, where strong attempts to correct one's taste is not necessarily directed from people with the "majority taste" towards the people with "minority taste", is when there is a choice between a "more masculine" and "less masculine" option, with the "more masculine" side expressing their preference in a much more "we are normal, you are the perverts" way. This hold even when the "more masculine" side is not in the majority, and it is most evident in the conversations about preferences for men with or without body hair, especially on web forums - in live conversation among adults politeness usually softens this phenomenon. It is quite common for women who prefer hairy men to say "I want a real man, and not a boy/homosexual/metrosexual". It is, OTOH, quite rare for women who prefer a hairless man to say "I want a real man, and not a gorilla", etc.

ObDisclaimer: I am not saying that most women who prefer tall/masculine/non-Asian/hairy men do this. I am just saying that enough people do this to make the phenomenon noticeable to the people whom it targets.

Anyway, the above was just a description, and the question is: what's in it for them, if anything, sociobiologically speaking? Does a woman who prefers tall men gain anything in the evolutionary sense by persuading a woman who does not care to pursue tall men as well?