My boy, girls don't sweat... they "glisten"
Unless your gf is on top the whole time, she's not gonna be working out as much as you are, since you're gonna be doing most of the thrusting. If you wanna see her sweat, get her on top of you and make her ride you (no helping from the bottom!) I bet she won't be able to keep at it for long though.
Cheers,
Rogue