I'm afraid to walk on into someone's room just incase I catch someone, then being all in the mood, they get ****** strong and start to do unsavory things to me.
But no, seriously, I live with my aunt. She straight up just opens the door and doesn't give any sort of **** when she does. She can, because it's her house and I never complain, because I really have no room to.
There are lines that can be drawn once you turn 18. It's common courtesy... respect... whatever you wanna call it. If I had a friend staying over whether it be for the night, or for a few weeks because they need a place to crash... I don't just barge in on them "because it's my house."