I think I'd question my view of the Universe if I discovered such things were possible. Then, I'd seek out my partner and see if he turned into a woman. Otherwise, I guess we'd be gay?
Then, I'd probably try playing with myself and figuring out how to use my penis to pee.
I'd have to figure out how to explain to everyone how I turned into a man, but I don't think I'd mind too much. I think how accepting I would be of the situation would depend on how good looking a man I was. I would be upset if I was an fat, pimply, ugly man.