This is the worst question from this thing I've ever seen.
First, I believe they were asking 'what celebrity would you have a sex change for?' Sexual identity is not the same as sexual preference. Just because you want to have sex with the same gender does not mean you can sexually identify with the opposite gender. I'm a gay male. I identify sexually as a man. To suddenly become female would cause a world of problems for me, as I kinda find vajayjay repulsive. I don't wanna touch it, much less have one myself.
Further, if you're going to have a sex change, it should be because you sexually identify as a gender other than what you are, not because someone else wants you to. If Brad Pitt will only sleep with me if I'm a woman, then I guess he'll be missing out. I'd gladly sleep with Lucas Black, but he'll have to be okay with me being a guy. As far as women go, even though I'm gay, there's still a few celeb women I'd have sex with, just because. Like Madonna, for instance. If she rolled up and said 'You. Me. Bed. Now.' I'd be like 'Yes ma'am.' because HELLO! it's Madonna! But I'm not going if she wants me to be a woman first. Why? Cause I have no desire to be one.
So, now that I've totally beaten the dead horse, my answer would be NONE. This was a poorly worded question. I should have just ignored it and moved on, but it pissed me off.