That is a very interesting question, one that is deserving of it's own thread. It's not relevant to this discussion about a crossdresser being with a man though.
The points I am trying to make are:
1) A crossdresser can't feel what it's like to have sex as a woman because he has no vagina.
2) Dressing as a woman doesn't make you a woman. It makes you a man dressed as a woman.
Now anyone is free to have sex with whoever they want and however they want. Just don't think that how you are dressed changes anything. It does not.