I lived with the same roomate for all four years of college, so that tells you a little about how we worked things out.
Our deal was: the person being asked to leave got to say what time to expect them back. And sometimes (finals week, say) it just doesn't work to be kicked out of your room.
Incidentally, the proper term for having been sent away for your roomate to get some is "Sexiled".
We had a loft in our dorm room--I slept above, he slept below. lurkette would often sleep over which usually started completely asexually and often ended up with my poor roomate getting fucked over.
He never did say anything about that. Hard to say if he was asleep or not.