I think it is normal and just goes to prove how much effort you are putting into having sex. It can be vigours exertion so its likely. My boyfriend has never mentioned anything, but he is usually more sweaty than I am, apart from when he gets me to wear my uniform, and thats just murder as its so warm. He has this thing about me wearing my full uniform, complete with blouse, tie, blazer, skirt, stockings. It get very very warm doing it for any length of time wearing all that!