Do you ever get the feeling that you went about this the wrong way...?
Top talking point between father and son, as your father I love you, no matter what, and the life of this family is secure.
The key player in this transaction was actually your wife, his mother. Let me explain.
You say this middle son (age unspecified, but absolutely crucial: if about adolescence any question about sexuality is important to him) often discusses your cross-dressing with his mother. Do you know the substance of these discussions?
Do you understand just how strong the peer group (your son's schoolfriends) are in any judgement? We bust a gut as parents, only to find that Joey's (uninformed) opinion counts for more than ours!
You seem far more concerned to stress to your son that you are not gay. Your son felt more secure discussing his concerns with his schoolfriends than he did with you. Did you not see an opportunity when your children discovered you dressed to reassure them that the family was still the same? Lost opportunity there.
So go back to your wife and explain that you have tried to discuss this with your son but no go. She will explain that much of this is to do with embarrassment; your son realises he's outed the family to the neighbourhood and feels torn between the schoolfriends' explanation and yours. You could try a three-way discussion or anything else she may suggest.
The bottom line is that your relationship with your son is more important than the differences between you.
Things could be worse. A 15yrs old male caller to a support line was distressed because, having discovered his father's cding recently, he could not understand why his mother had allowed it. (as he put it!) He was apopleptically angry with the mother and asked the female manning the helpline to explain it to him.....