Mikusagi
If you want to talk to them about it, but feel too embarrassed, you could always look for their condoms and through them away (not in the trashcan, they can find them there). They might understand that you feel uncomfortable then xd
I mean it could be another way of telling your opinion. That's true; it could be a way to let them know that they're being too loud, but I really think throwing their condoms away would be a bad idea.
1. Condoms cost money and are really important. It's never a good idea to tamper with someone's condoms. They could get caught up in the mood and decide how to have sex without a condom if they can't find them. There's nothing funny about accidental pregnancy and STDs.
2. Snooping through Mom and Dad's room isn't really respectful. I don't want people to snoop through my stuff, so I don't snoop through theirs.
3. There's a very good chance her parents are not using condoms. When people have been together a long time and don't have STDs, they usually end up opting for another form of birth control.
There's nothing wrong with the fact that her parents have sex. Her issue was more that she was able to hear it, which we all agreed can be avoided for the most part while still being safe and respectful. Or to get the message across that they're being a little loud, she can turn the tv up really loud.
xp That would block the sound for her and be a hint to them that she can hear them!