"Why wouldn't anyone want to chose to be gay?"
Let's see...alienation, fear, discrimination, missing the prom (maybe not so much these days), hiding your feelings for someone, torturing yourself throughout childhood as you try to "fix" things yourself...etc...etc...etc...
I'd much rather be married and starting a family like all of my straight friends and family. I don't see a lot of happiness in the gay world. I see a lot of WANTING to be happy, convincing oneself that one is happy, but not a lot of TRUE happiness.
I wouldn't undo my sexuality now. It has made me who I am and I'm happy with myself. But, the suggestion that anyone has a choice is pretty ludicrous in my mind. I think you have a choice in how you live and respond to your sexuality. Personally, I can't deceive myself enough to believe I'm straight or even "bi." And I certainly could NEVER deceive another person, such as a wife or girlfriend, as I try to have my cake and eat it too. So we have choices - and we can choose to make good, honest, self-actualized ones.
But we can't choose our sexuality.