At 18 they can tell you to fuck off.
At 18 you can tell them to leave.
Personally, If my child wanted that same scenario I'd tell them "no" (No staying over, no fucking, no nonsense). Then again, I was raised by parents who were kinda old school.
You want me to pay for you when I don't have to anymore? You obey my rules regardless of how crazy my wife is.... The breadwinner (hopefully that's you) makes the rules.