I really hate to do this...
I'll jump in on the issues because hell, I like to discuss things.
Evolution: From a limited perspective it is a perfectly functional theory. Things change, arguing they change for the better isn't much of a leap. However, evolution as the explanation for life as we know it on the planet is a huge leap with almost nothing to support it. It is not even a theory, it is just hypothetical. The fact is we do not know how we got here, we can just guess and no religion nor theory has provided a explanation I consider valid.
If God made everything how did God get here? If everything came from a giant mass that exploded, how did that get here? We want to pretend we know everything, but on those matters we really know nothing.
Homosexuality:
I'm far from a expert on the matter. But, I did do security in a gay club and I have been friends with a few gay people. So, I would like to think that I am not simply speaking out of ignorance. I think that people who wish for homosexuality to realize more acceptance have been pushing far too hard to classify it as a biological thing. If they can classify it as biological and not a choice, then much like race they can argue that any bias against gay people has no basis other than bigotry. While I admire the goals, I think they are clouding scientific reason.
I do believe that biology can play a part in homosexuality, but even from that perspective it would be a mutation. I think it is a minority of the cases demonstrate ample proof of it being genetic. For example, hermaphrodites or people exhibiting secondary male or female characteristics (female for male or vice versa) obviously have a very good case for biology playing a part.
From my experience though, development and deviance are the primary factors. It is obvious that sexual activity can be affected by trauma of one sort or another. Take for example two females I know and how their actions were influenced. One is my niece, she was abused when she was around 12 and has been sexually active (with males) since then. She is now 15 and is out of control, she goes from one boyfriend to the next often daying men over 18. Her mother (father is divorced, another factor) has little control over her. Anyway, for example over the past year she's probably had 5 or 6 boyfriends and most likely been active with all of them. A friend of mine growing up, had her father die while she was relatively young. Looking for a father figure, she dated at a very high rate and I'd say from my knowledge she was sexually active by 8th grade (at least). Once again dating men 18 and older and litterally dating innumerable people. I personally knew 6 guys she dated (mind you we didn't go to the same school or anything). I'm talking about her dating probably 100 guys by the time she was 18. Obviously not normal. I met a guy from her town once, I was like hey do you know her? Yeah, I dated her...
Ok, now how does that compare to homosexuality? The first gay guy I knew (read: knew he was gay) was a man who had been orphaned and raised by a priest. Do I really need to say more? The other gay man I knew best was someone who had been raised by a abusive alcholic father. My brother's first wife decided she was gay after a few years of marriage. I mentioned this in a chat room once, only to have someone inform me that you don't "decide you are gay". Well, she did. My brother also dated a bisexual chick, I didn't know her well enough to comment but...
Bisexuality is something that flies in the face of people that claim homosexuality is not a choice. I think bisexuals tend to be more deviant than their homosexual counterparts (in other words they'll

anything because they looovee

ing, not all mind you). They have a choice and they choose both, something which is very hard to explain biologically (unless of course they are a hermaphrodite).
So? What am I saying?
We can't prove evolution as a explanation for our existance. So, I think we should focus less on trying to make it "fit" and leave our minds open to all possibilities.
Homosexuality isn't a choice in most cases, but I'd say it isn't biological in most cases either. We don't choose the facts that affect our development, but from my experience it is mostly a issue of development and most people that say otherwise seem to have a agenda.