Hmm well Carla ducked out on the age answer

But not vitally important. Unless of course you are young enough to still be stuck doing your parents bidding and then I suppose that is out of your control to a point.
Regardless of your 'parts' you seem to describe activities meant to be 'girls being girls', and it sounds like your dad simply can't relate to his 'son' having girls over for a girls gathering. It sounds like he's in a sort of sense of denial. Just guessing there, but his constant checking sounds obsessive and sounds like he really doesn't really believe in you.
It's nice you have gal pals ok with you being one of them.
Do you not have alternative choices of where to gather? Surely the girls must have an improvement on your father's behaviour.
It sounds very unfortunate the way your day is acting.
Myself, my son is 19, that makes him an adult, I have I hope made it plain to him it's his room and what he does in it is his business, thus him lately having his girlfriend over and her spending as many evenings sleeping in his bed as she does him sleeping in her bed at her place has not bothered me.
I am not sure of their sex life, and it's none of my damned business, he's an adult now. I'd only object if they were having loud sex in the middle of the night

Not all parents though have much respect for their kids own rights. Too many parents are all about control, just because they think it is their right to just run their kids lives. I prefer to be a voice of advice, not demands.