What happens if I marry a transsexual and I didn't know?

The odds of that happening are very low, and in the case of many post-op trans* people (especially in the cast of male to female), you would never have any visual cues that your loved one may have had such a past.

In cases where you absolutely cannot stand the fact, such a revelation could be grounds for divorce. But I would hope that if you loved a person enough to get married, you would love them enough to deal with their past. After all, it is that past that turned them into the man or woman you love today.

That's not to say it would not be weird, and that is why such secrets are often kept. But if you worry about a reveal of that magnitude, just be sure before the wedding to sit down and say something like "We are about to be man and wife. All secrets on the table. I can take whatever you can say, if you can take whatever I say. Here, I'll start: I once had sex with a trout. Your turn." Such a conversation would be good for any marriage, as secrets can't stay hidden for long in such close quarters.