I think this is the issue.
I have a couple of bits of advice
1. If you are going to share body fluids with someone, even if it is a one night stand, treat them with respect and make sure they know where they stand, it is a monogamous shag buddy thing, open shag buddy thing, one night stand, potential relationship (cause this one catches some people out as well)
2. Never do or say anything that you would not do with your partner/shag buddy standing beside you
3. Never do or say anything about your partner/shag buddy that you would really not like to have done to you.
Pretty simple, it does take a bit of courage to be brutally honest and have integrity at times, but it really does make life SO much simpler and easier to deal with the fall-out if there is any.