It's not all that unheard of Original Poster. I believe the proper terminology is voyeurism. Most of the world is a bit too insecure with the idea of our special one with somebody else, but its quite a kick for a fair amount. If I had to imagine what stirs them, i guess kind of like watching real live porno right in your very place, and its somebody you know very well?
Certainly, this doesn't count out the possibility if he does have other motives (i.e. you end up having sex with other guys, thus he can prep to ask you to have sex with other girls, or if he is "testing" you). But this scenario is not all that weird, so he just very well mean what he asks.
Instead of focusing more on what he wants, considering that this very well involves you, do you want to do this for him? If you don't feel comfortable with the idea of that, then he will just have to accept it.