The cross dressing is a red herring for me.
The key issue is that you quite rightly had boundaries about dating sites and sexual activity with anyone other than you. In a marriage you should expect those boundaries to be respected.
The fact that your husband has put his wants above your needs means it's him, not you, that have thrown away your marriage. Even if you did stay together you don't have the marriage your son needs as his example.
I have certain things I would like to include in my sex life, DH has certain things he'd like. We don't include them because the partner doesn't want to, simple as that!
It doesn't matter what the issue is, if the other person can't live by those boundaries they should at the very least do their partner the courtesy of giving them the choice whether to allow. If you love somebody, that means you put their needs at least on a par with yours, if not above.
The fact that your DH placed more importance on his activities shows he doesn't value or respect you as much as you deserve.
Lastly, my parents stayed to get her for my sake and it did not give me a great example of what marriage should be, despite their best intentions. I have had a number if issues as a result before mostly working through them and finding my OH. Even so, I'm not the person I'd like to be for my DH because I've not entirely been able to deal with things.
Living a lie, be it your husbands to supposedly save your feelings, or your joint lie for the sake of your son, may not be the best thing.
If you stay, do it because you want to stay in the marriage and don't try to justify it as being in your sons best interests. Having been the child that happened for I can tell you it's not the great life you want for him.