These are all the different possible reasons listed in no particular order:
1.) Genetics Only - I personally don't agree with this until I see more research done on it. It would imply that homosexuality is hereditary, which I've never seen proof of.
I don't see this as being true - could be a recessive gene, or just a common mutation
2.) Personal Choice Only - I also don't agree with this. I think this may be true to a very small extent, but surely only in a minority.
A very small minority....I don't know of anyone who has "chosen" their sexuality. If there was a choice, it's more likely to be in the sense of people in denial of being gay.
3.) Combination of the 2 - This is my winner. IMO this is not solely a preferential issue. I think there is a genetic predisposition to homosexuality, but I also think that a choice is made whether consciously or subconsciously to actually become homosexual.
4.) Outright Horniness - This is for all the bisexuals. I don't believe that is genetic at all, just you all being greedy for some nookie! 
I don't see why it can't be genetic
John Maynard Keynes Capitalism is the astounding belief that the most wickedest of men will do the most wickedest of things for the greatest good of everyone
Patternweaver's fiancé
Denice
KRO
*bowtieless*
*NM*