I don't think that when giving advice that the phrase "honesty is the best policy" can be repeated too much. If you are in a relationship and either want to come out or think you have been caught out then being honest may certainly cause heated discussion but I would always say that this is preferable to being suspected of far worse than wearing female clothing.
In all honesty, if faced with the situation where you have to front up to being a crossdresser or on the trans spectrum rather than being branded an adulterer or worse then the former gets my vote every time. At worst your gf/partner/wife will end the relationship, however most will at least talk and attempt to understand why.
The ideal situation is to avoid this by being totally honest and discussing the matter in an adult fashion.
"Denying who you really are, only leads to regrets later in life. Seize the moment because it will never come around again."
I have been reliably informed that I am a much bigger bitch when I am NOT Sara!!!!