For a start, because I think people choose to be who they are. I believe the quote goes:
"We cannot choose who we are, yet who are we, other than the sum of our choices?"
So it's not 'choose to be who you are' in just spontaneously deciding to be one thing, but that a person has a personal responsibility for all the life choices and decisions that led to him being who he is today.
And I think sexuality is part of that very, very complicated make up. I think it is likely that some people have a greater genetic disposition to be homosexual, but only in the same way that some people have a disposition to be good at music; it still doesn't define who you are and what you do. The way you live your life does that, and your upbringing, and your reactions to that, and your general outlook on life- that is most certainly a choice- are all part of a complicated web of conscious decisions you make.
Sexuality comes out of this. Personally I cannot accept that it is simply something set at birth that cannot change.