Hollywood (or, more specifically, the big six studios [http://en.wikipedia.org/wiki/Major_film_studio#Today.27s_Big_Six] that can put a film in more than a couple hundred theaters) would not make a "yaoi movie" without the gayness being the only thing it's about because for all the talk of "liberal Hollywood", the vast majority of those with power are very prejudiced, intolerant, and cynical. If you watch more than one or two films a year, you'll notice very quickly they're attached to the idea of the white American heterosexual cisgender male as the default viewpoint for everything and refuse to normalize[footnote]When I say normalize, I mean presenting a situation as an accepted fact rather than a plot point. An example would be how Alien normalized the idea of a woman as a warrant officer and respected member of a ship crew by showing her as that without anyone commenting on her gender.[/footnote] much of anything else. To them, homosexuality is a joke, a threat to a straight protagonist, or a "serious issue" that brings good publicity every couple of years.
Also, as seen in the documentary This Film Is Not Yet Rated, the MPAA ratings board is notoriously homophobic and slaps higher ratings for merely involving homosexual characters. (There is some grey area there because they were also shown to give independent films higher ratings as what amounts to an anti-competitive form of censorship.) The empty-nest parents making up the board would fight the idea of a movie about gay men targeted to t(w)een girls no matter how tame its content is.
