Oh for goodness sake, FloraFox, you've seen me on the feminist boards, you know I don't for one minute think that women should be handmaidens or subservient in a relationship. Nor do I think they should just accept anything.
BUT, not putting up with that with they are not comfortable should take the form of either having a discussion with their partner and coming to a compromise which is acceptable to both of them, or ending the relationship. Not telling their husband to repress an important part of himself if he wishes to remain in the relationship. You would, I'm sure, find it completely unacceptable if the roles were reversed and it was a man laying down the rules of what was and wasn't acceptable to him in terms of his wife's behaviour.
Yes, he should have told her before they entered into a long-term commitment, and yes, certainly he should have told her before they had a child together. We don't know his reasons for not telling her, but shame probably had a lot to do with it. I agree with you 100% that he should have told her and was wrong not to. However, that was 20 years ago, and two wrongs don't make a right. She is equally wrong to tell him he can't behave in a certain way. If she wasn't prepared to accept this side of him, she should have ended the relationship 20 years ago, child or no child.
And he should have ended the relationship instead of accepting her cruel treatment of him by forcing him to keep it repressed.
To sum up the original question, is the OP unreasonable to not want her husband to wear dresses? In my opinion, yes, but that's my opinion. She is entitled to her own.
What she is not entitled to do, however, is tell him he can't do it. He is just as entitled to live his own life and pursue his own interests as she is.
So IMO her choice is either to accept him as he is or end the relationship. She can't continue to force him to be someone he isn't, just to placate her own sensibilities.