My boyfriend spent the night a few days ago and my parents were OK with it.
But we decided to have sex once my parents went to bed. But the next morning, my parents were asking me why I had sex and they said they could hear me! Coming to think of it, I was being quite "vocal" but I thought they were asleep.
Thats hilarious. Their faces would have been priceless.
On a more serious note, just explain that your not a slapper and that you're sorry if it bothered them. Be as blunt as that. I find it usually helps if it's not something that can be undone. Time will, as others have said, heal the wounds.