Logically, being gay or bisexual are as much a choice as being heterosexual, which is to say not really at all. While I think both genetic predispositions and environment play a part, I also think that at no point is it a conscious choice. If I were to give a weighting, I would weight nature as a more important factor than nurture.
Furthermore, I have quibbles with the conception of sexuality as some kind of set of which a person can be gay, straight or bisexual. (This kind of conception where there are three clearly demarcated boundaries also seems to ignore trans-preferences?) I conceptualise of sexuality more as a continuum which one exists on and can move about, but that one does so unconsciously and the degree to which one moves on the continuum is probably quite limited (e.g. you’d be unlikely to get a hyper-straight person moving to hyper-gay or even through to bisexual). Added to this, there’s probably another continuum which ranges from hypersexuality to asexuality, which a person will also move along and which will determine how important a person’s sexuality is to themself (not necessarily other people, sadly).