I think it's absolutely not okay. I think it's disgusting. My mother used to leave her bedroom door open at night and I could not only hear but lie there and watch her having sex with her boyfriend. As the above person said it can be fear inducing, because at first when I was only hearing it I had no idea what was going on and thought something was wrong (it's probably funny now, but at the time I was terrified), but once I had worked out exactly what they were doing it repulsed me. I would wake up at 3-4am and have to lie there blocking my ears for half an hour so I wouldn't have to listen then, then I couldn't sleep afterwards because of what I knew they'd just done. Once they were doing it at around 6:30am and I was freaking out the whole time because I had to get up for school soon and was worrying my alarm would go off while they were in the middle of it (while both our doors were open).

Over 15 years later it still makes me sick.