ZiMsTeR saidI fancied him until I found out he was actually gay. Why is that?
I really dunno. Rather than offer a load of amateur psychology, I'll just say in contrast that I'm the exact opposite, and your reaction does puzzle me.
I may find a straight man physically attractive (beauty is beauty), but I don't waste my time fantasizing. My attitude towards a gay man who's out is totally different, and I feel an immediate bond with him on many levels, including potentially sexual. Learning for the first time that a particular man is gay makes me like him more.
And especially if the man is known publicly in some way, I'm proud of him for being out, hope he'll be a good "ambassador" for us. I think that the more gays are out to the public the better for gay rights, and for my own social standing in the world (although I don't support forced outing).
So being out makes me like a guy more, not less, and I wonder if you have any ideas at all yourself why you feel otherwise.