Just go.
You don't have to tell your mom that you've been having lesbian sex.
You can always find a bullshit excuse to ask her to leave the room, like whoever said the bit about being uncomfortable. Worse comes to worst, you can just say you were masturbating. She can't really hold that against you. Although it might be a little awkward.
EDIT: Link
http://www.uristat.com/shorthed.htm
My girlfriend got an UTI after a night on the beach and too much sand. So we got that and it cleared it up fine.