This is probably going to get me lambasted, but you know...I gotta say, the more I think about it...the more the choice and genetics/biology routes BOTH kind of irk me....
I recently asked a bi friend a question similar to OP's (without the supporting logic/explanation) and he got pretty irate and basically said that no one in the LGBT community woke up one day and decided who or what they were going to be attracted to. I ended up launching into a pretty long argument, adapted below, because while I don't disagree with what he said, pulling the chemistry, biology, or genetics card instead, to me, has always felt like nothing more than an excuse.
I don't mean to say either the choice route or the genetics route is or isn't true, but at the same time, it doesn't do anything positive, it doesn't really open up any sort of dialogue about the stigma of being different, it's just "this is how things are, ha ha ha, moving on..." or in the case of Arnold as the Terminator in T3 after picking the line up from that male stripper at the club on ladies night: "talk to the hand".
Personally, I don't think I could say WHERE sexual orientation comes from, nor do I really care, but I do agree with the OP that there is a lot of choice involved... "Do I choose to persue that person?" "Do I choose to accept this person's invitation?" "Do I choose to tell my family I'm in a committed relationship?" Choice, just not necessarily choices about attraction itself.
The reason that choice versus biology matters--and in my opinion: the main problem--is in the stigma of being different, of deviating from what is perceived as the so-called "normal".
I think that a lot of the arguments surrounding the choice route come from the religion side (on at least one level or another)...for which I'll point to my personal belief on the problem religion has with homosexuality, or really anything sexually deviant. It's all sex for pleasure, not sex for making more followers (i.e. babies)... Making new followers for a given religion--so strictly speaking, the actual act of sexual reproduction--is something that's a whole lot harder to accomplish if it's two guys together, two girls together, or a lady in red stringing a guy up from the ceiling and working him over with a feather duster and/or cat-o-nine-tails.
IceForce said:
Do you hear people talk about "acting upon" their heterosexuality?
Not normally, no, because heterosexuality is considered "normal". For another fun facet of normalcy, look up "left-handedness" and whatever variation for "satan" you prefer in the same search.
This is all personal opinion, I realize, but the following more so than what's preceded. So please don't light your torches until the end.
Going back to my view of 'genetics as an excuse', I feel that it separates you from the problem. It almost sounds like "hey, this is a part of who I am...I know {it's wrong/you think it's wrong}, but I can't help that it's a part of me".
Having all that outside pressure, with many thanks to religious views, social/cultural standards for normalcy, or a combination of the two, I feel builds a subconscious platform of acceptance that there is something wrong (either that something might be or is sinful, or simply the idea of "different" as a negative). You may or may not be consciously aware of it or want to be a part of it (think of the fashion industry and the whole issue surrounding portrayals of women in TV, advertising, etc.), but it's still in your head, so there's a need to defend yourself, to point to something unchangeable as a way of shifting blame. Thing is, if there's any blame to be had, that doesn't put the blame in the right place.
The problem with sexual orientation is not in who you ARE and where that orientation comes from, it's in how and why you're PERCEIVED by others and, by extension, how you perceive yourself. Putting fault or crediting something such as choice or genetics for our own sexual orientation doesn't do anything to help anyone.
A long time ago, a friend of mine and I were at a local pond swimming. I noticed a leech on the top of his foot and when I pointed it out to him, he flipped for a few minutes--complete with wordless screaming and wild flailing--before finally sticking his foot in the sand. He immediately relaxed and told me, "It doesn't hurt if you put your foot in the sand." If you know anything about leeches, you know why that assumption is wrong. He thought it hurt because he thought it SHOULD hurt. In a similar manner, it's offensive to point to choice for homosexuality or bisexuality or what have you because people think it SHOULD be.