I've been with my gf for ages now and have been having sex with her for the majority of that time, I always assumed my parents knew the ammount of teasing I got off them.
Then one day my mum just put me on the spot and asked straight out I was pretty suprised to be asked - much like a dear in the headlights. I answered truthufully got a birth-control talk and they're cool with it, I just get more jokes from them about it.
She lives with her aunt and uncle who also know - I stay overnight there regularly in my gfs bed and they are fine with that, her aunt did walk in once which spoilt the mood to say the least.
My gf isnt allowed to stay over-night at my place I just get stonewalled when I ask for reasons and I'm moving out in a month or so, so while irritating that's bearable.
I do however have two funny stories:
Firstly when drunk my gfs aunt asked her whether she used the pill, she said yes, her aunt asked if we used condoms she said yes, her aunt advised against it as without is so much better.
A friend of mine (also been with his gf for a while and having sex for almost a year now) was in the kitchen talking with his parents (his gf wasnt there she was away or something) then his dad has stood up said to his wife (my friends mum) "lets go to the bedroom and have sex". My mates just blurted out something about rubbing it in that he wasn't getting any to which his dad replied "I dunno what you're complaining about - you get it three times a week, I'm lucky to get it once a month".