the man who is pregnant

So on oprah the other day, she had a guest come on that was born a woman, but had a recent sex-change. That wasn't all that was discussed on the show, it turns out that the man was pregnant. This was not one of those rumors going around the internet..... also, she was legally married to a woman not too long ago. What i don't understand is why gay marriage is not legal but if technically he was a woman, that he got to get married to another woman?