There are two cats living in my apartment. Only one of them is mine,
the other belongs to a girl who is subletting for the summer. Her cat
is not very well behaved, and can't be trusted not to pounce on mine,
so we've been keeping them separate all the time. My cat stays in my
room, and her's gets the rest of the apartment unless he misbehaves.

For the last two days he has managed to get into my room and bother my
cat (we think he's been trying (succeeding?) to have sex with her). If
the closed door won't stop him, are there any other tactics for
keeping him out?

I feel awful that I can't keep my cat safe and comfortable. At least
the other cat leaves in a month and a half.
-E