I've been thinking about the uncomfortableness too, and the best I can come up with is this: I think *my* issue with it is that when I'm staying in someone else's home (which I am now, I'm at my mom's for the month of October), I would NOT be having sex on her sofa or anywhere else that wasn't locked and private (like when no one is home).
Just because I don't feel that sex is something to be shameful of doesn't mean I have to be comfortable with everyone knowing when and where I'm doing it, you know? To make my thoughts even more convoluded, my 5 year wedding anniversary is coming up in a week and a half and when my husband arrives we're getting a hotel room for EXACTLY this purpose, to get our freak on.
My mom knows this, my sisters know it, it's all out in the open. But for someone reason getting in on after having not seen him for a couple of weeks with her and my cousin in the next two rooms over is, well, weird. And I grew up in a pretty liberal open family. Sex is all good if it's consensual and safe, however you like, with whoever you like it with. Masturbation, pre-marital sex.. nothing was off limits. But getting it on while everyone is downstairs watching TV, or worse, making them listen to it if they were in their own rooms, is just.. weird. I can't explain it, maybe it's just me imposing MY comfort level on others? If my mom or my cousin (who is also here right now) were to bring someone home and basically force me to listen/be aware of them having sex, I wouldn't be too happy about it. They can do it 'till the cows come home, but it really doesn't need to be a public affair. And it would be in this house.
ETA: Thinking about it a bit more, I think my discomfort about being too
open stems from the fact that my family really was open about sex. Perhaps too open. I remember one time we were on a sailing trip and were anchored out and I rowed to shore to take the trash to the bins and when I got to the docks and looked back at the boat it dawned on me that ALL the other boats were as still as statues, and ours was a rockin'! The guy that owned the marina introduced himself to me and asked which one was ours and I was MORTIFIED that my mom and step-dad were having a quickie while I was gone and making it blatantly obvious. LOL. I guess I'm scarred for life.