I've hosted many couples and have luckily not had to deal with this sort of thing too often as most people know to be discreet in a shared home situation.
Early on, I had one couple who, on their first night here, had a loud and prolonged argument until 4am (one of them was having an affair). After that, no real problems of this sort. I've had many noisy guests, but they were noisy in other ways.
Recently, I had a solo guest stay and ask if her 'friend' could stay over for one night. I agreed. She brought him home, briefly introduced him to me (which was polite) and then they went to her room. As it was not particularly late in the evening, I went up to my room, which was opposite hers.
She would have heard me come up the stairs and go into my room. I was tidying it up and had the lights on. Still, that didn't prevent her from having noisy sex, knowing I was only a few feet away. I was pretty surprised by that to be honest. Still, I didn't say anything to her about it. It would have just been far too awkward. She came from a culture where people tend to be very shy, polite and quite conservative.
At the same time, had a young couple here for a long term stay. They were obviously very 'loved up', but in the three months they were here, I never once heard anything that would make me or other guests uncomfortable. Like I said, most people do appreciate that it's a shared house and that it's not okay to subject other people to the sound of your 'honeymooning'!
Unfortunately, these things do happen occasionally. I think one way to prevent it is simply not taking one night stays (or at least not one night stays for couples). I have recently reduced my max occupancy for each room from two guests to one as well. That works for me as I mostly host solo, long term travellers. I can see that it might not be economically viable for hosts that offer short stays.
