siamesecat's story reminded me of one. Waaaay back in my uni days two friends and I were renting a house together. Since it was so convenient to town often friends would crash in our lounge room if they couldn't get home safely. We had no problem with this - the more the merrier!
Until the night that one friend bought a partner home with him. To our lounge room. The lounge room through which we had to pass to get ANYWHERE in the house (including the bathroom and kitchen) from our bedrooms. So we were essentially trapped in our rooms unless we wanted to see something we didn't want to see. One of my friends was eventually desperate enough to go to the bathroom that he *ran* through the lounge room, averting his gaze and mentally going "Lalalala". He heard enough to confirm our suspicions though. Mind you, this was the next morning so they didn't even have alcohol as an excuse. Yeah, he didn't stay at our house after that.
Of course these days I'd be more likely to just interrupt and kick them out but we were fresh out of high school and all raised in fairly conservative families so we had no clue what we could do.
I have to admit I wasn't this "polite" when my housemates and I ran into this problem.
Three females and I (all college students at the time) shared a house. Three of us each had a bedroom in the house and the fourth, whose parents owned the house, had the guesthouse in the backyard. We had a party, and one of our guests (who was also a friend and fellow college classmate) wanted to be alone with his girlfriend in one of our bedrooms as they both still lived at home (we were all either adult teens or in our early twenties at the time). I said no (I recall my answer being something like, "If anyone is going to be intimate in my bedroom, it's going to be ME so, sorry, but NO." I told him he could try asking the others and didn't think anything of it until near the end of the party.
I was too drunk to drive and some of our guests needed a ride home. GuesthouseHousemate said she was fine and would give everyone a ride; she just needed to retrieve her keys from her room so she went out to the guesthouse to do this. A couple of minutes later, she comes rushing back into the house.
GH -- "David and Donna are in my room making out!"
Me -- "Oh, you actually told David he could use your room? The rest of us all told him no."
GH -- "NO! David said he needed to talk to Donna alone and asked if he could use my room for privacy so I said yes."
Me -- "Yeah, David was lying. David asked each of us if he could use our bedrooms to play scrabble with Donna, and we all said no."
GH -- "Well, my keys are in there, and I can't get them."
Me -- "I'll get them. I don't care; we need to get these people home. Where are your keys?"
GH -- "They're on my dresser."
I went out to the guesthouse and could immediately see what was going on through the door (it was a glass-paneled door with sheer fabric over the glass so you could see in easily when the light was on in there). While David and Donna had not quite reached the full-on scrabble stage (they were fully dressed), they were lying on GH's bed and kissing pretty intensely -- David's hand was clearly somewhere I didn't need to see. Now, keep in mind, I was really drunk so, if sober, I might've not been so "brave." I actually didn't pause once I reached the door. I saw what was going on the moment before I charged in. I averted my gaze, said "excuse me," grabbed the keys and turned around leaving immediately. I went straight back to the house and handed GH her keys. Everyone asked me if David and Donna had stopped, and I said, "Oh, no; they were still fooling around." Right after that sentence came out of my mouth, the sliding door to the backyard swung open and Donna came charging through the house, through the room without a word to us and out the front door. Her face was stormy, and she didn't look at any of us. David was a few steps behind her and didn't say anything to us as he followed her out the door. We all just looked at each other and started laughing. I think I just shrugged and said, "Well, they never had permission to be making out in anyone's bedroom; if they want privacy, they can make out in his car."
I don't think we ever saw her in our house again. David definitely didn't try to use one our bedrooms again as his scrabble arena. I talked to him about it later because I was wondering if she were angry given the stormy look on her face (not that I cared; I was just curious). David said Donna was angry at HIM but, otherwise, extremely embarrassed and wanted to leave the house ASAP (I told him she should've been as he had lied to GH in order to use her bedroom). David, oddly, wasn't mad at me (again, not that I cared as he lied to get what he had wanted). That was 18.5 years ago (and I'm FB friends with David so, no, I don't think he ever held it against me as we were always just fine after that).