Most people have no idea why they have the feelings that they do. When they fall in love, it's all 'chemistry' to them; they have no idea where the desires come from. Mostly, it's from genetically influenced hormomal reactions and the experiences they've had in their life. Women are 'wired' to be attracted to, and find sexually desirable, males who display what is referred to as 'alpha male' behavior. Men wearing female attire or behaving as a female kills any attraction for >99% of women. And as women rely on men for protection, it also makes women feel insecure when her man is suddenly revealed to be less of the masculine person she initially thought he was.
As far as your friend's SO not wanting her man to crossdress, well, sometimes we do things to help us maintain the illusion that 'everything's just normal'. Lots of people keep their 'head in the sand' about many subjects because to accept the reality, would make them feel insecure. So they prefer to remain delusional and keep thinking everything's fine.