I can only speak for myself on this issue, but as someone with SA/SP I do have an incredibly hard time approaching the opposite sex and do find it easier to approach other guys and start conversations with them. I can sometimes strike up a conversation with a girl, but I never know what to do when it comes to moving beyond conversational formalities. With guys, on the other hand, if I feel like they are receptive to a certain level of friendship then I can relax a little knowing that the social standards generally don't expect any development along certain expected lines (like with the opposite sex, you always get asked "what's the status of the relationship," as if it should either remain definitely Platonic or otherwise become romantic but nowhere in between). So recently in my life I've come to think that I'm probably bisexual because I find myself thinking about other guys a lot (for a few years now, and in more than just a friend-like way). I think girls are attractive, and I can see myself possibly getting married and having a family someday, but I can't rule out the fact that being very shy or having social anxiety might be a factor when it comes to being attracted to guys. With a member of the same sex, for example, there is less pressure to develop the relationship as expected by society. Also, as a very shy guy, having a relationship with another guy is attractive because societal standards often assume that women take a more passive role than men whereas that doesn't play in the favor of shy guys. I don't actually think my SA/SP causes my same-sex attraction, but I do think that it might make it easier to prefer guys over girls when I have an attraction to both. Then again, maybe I'm just more gay than bisexual... I'm still trying to figure it out myself.
Sexuality is probably genetically determined to some degree, but also determined by chemicals in the womb and possibly by environmental development as a young child. It's not inconceivable that the same pre-natal or environmental factors influencing sexuality could also play into social anxiety or other personality factors... though such cases might be largely incidental.