Body building and cross dressing doesn't seem much opposite, unless you classify as to body build for pure muscle on top of muscle kinda way. Body building for lean muscle is the best way to sculpt a body in my honest opinion.
But to the point of tucking, testosterone, the many other t's involved if any more. From my understanding is that tucking does not affect testosterone production, but just as was stated is that it does affect the sperm production as per body temperature and the testicles being basically inside the body rather than more of being in an external pouch.
I know for me, I feel more feminine when I tuck and can present myself a lot more freely as female. Even in the slightest of tucking slippage, or when just waking up in the morning and finding myself un-tucked, I find myself in a state of mind in which would be more male output in life, which sometimes I just can't stand at all. I think for me its a bit of a psychological type issue for myself, because I just want to get things moving faster than they are going.
I hope this helps.
Kate <3
