The thing is, homosexuals can be attracted to heterosexuals. For instance, there's a large section of porn for gay men dedicated to displaying straight men in sexual situations, usually with other men. (Source: http://www.amazon.com/Billion-Wicked-Thoughts-Largest-Experi...)
Yes, some homosexual men have been attracted to heterosexual men (and some heterosexual women have been attracted to homosexual men, etc.), but it is not the norm at all.
And yes, there is some porn aimed at gay men that shows 'straight' men engaging in gay male sexual acts, and there is porn aimed at straight men showing women engaged in same-sex sexual acts (just look for "girl-on-girl"), and there is porn aimed at straight women showing men engaged in same-sex acts (just look for "slash-fic"). The vast majority of porn consumed by gay men is gay male porn though.
It is completely wrong to presume that heterosexuals are constantly at threat of advanced from gay men.
