Or do you mean selection pressure in the looks of catwalk models currently being used ?
Obviously that is what I mean. We are talking about runway models after all, not the global human population. You are claiming that there is a selection pressure for androgynous body types, and that this is a result of the sexual preferences of gay designers. In that case, an obvious question is "Why not select for androgynous faces too?". A next obvious question is "Do these designers show any marked preference for young boys when choosing sexual partners?"
Another question would be "Given that the trend towards near-anorexic models has only occurred over the last couple of decades, and given that gay men have been in the fashion industry all along, what has caused the change in models?"
If your "theory" can't account for these factors, then it's not seeming very robust. I have no doubt that gay men have a substantial influence within the fashion industry, but when analysing problems with that industry we need something a bit better than "ZOMFG, teh fudgepackers are running the world and are taking over our brainz!"
As far as I can tell, although the "they want them to look like young boys" line may be obvious and convenient for some, it doesn't really seem to stand up to scrutiny. It looks to me like a better explanation is required.
If thjat is the cse then we have had our first male model model female fashion on a high-profile catwalk. How long before some designers uses all men, androgynous enough for you? We will ahve to seee what the future brings.
IOW, no real evidence for it yet. Ok.