One of my oldest friends, we'll call him Ted, had parents who were both alcoholic lawyers (both were having affairs it later turned out) who were almost never home, so their big townhouse in Central London was the go-to party house in our group.
He came back from college one summer with his new girlfriend, "Lucy" for the sake of the story, and naturally we had a house party at his parents' place. There were probably only about 30 people there, but the place was fucking huge, so plenty of rooms to peel off to if you wanted to.
Around that part of Central London, there are also a lot of sleazy hotels (think the hotel where they do the deal at the end of Trainspotting). Nowadays they're probably full of illegal migrants on the taxpayer tit, but back then they were for junkies and backpackers. My friend's brother, let's call him Dick, went to pick up some weed and came back with this young Aussie guy (one of the backpackers I guess). It was random, but the guy was almost moronically cheerful, like a lot of them are, and seemed completely harmless, so was let in to the house. He happily attacked the free beer and pizza and, I at least, paid him no more mind.
A few hours later, Ted was wandering around drunkenly asking everyone he came across if they'd seen "Lucy". No, was the resounding answer. Dick, through his white widow haze, had a moment of premonitory clarity and said "where's the Aussie?" We went up to the top floor, where there were three rooms - Ted's bedroom, a bathroom and a smallish spare room that seemed to have no other function than as Dick's smoking and wanking lair.
I should point out that, despite the house's massive size and desirable location it was, inside, a shithole. After all, the two parents were rarely at home and were boozebags.
Well, you've guessed it. Inside Dick's masturbation furnace, there was "Lucy" on all fours, naked from the waist down, fully clothed from the waist up, while Crocodile Dundee merrily pumped away at her doggy-style.
A lot happened next in a very short time. Ted howled in outrage, turned away and ran to his room next door. His brother Dick turned red with rage, yelled "you fucking cheeky cunt" and attacked the Aussie guy, whose boner quickly disappeared. Me and my other friend ("Frank") just burst into drunk and stoned laughter.
The Aussie guy was very apologetic and insisted he had no idea Lucy and Ted were together, which calmed down Dick a bit, who then gave him 60 seconds to get out of the house. We went into Ted's room, where he was crying on his couch. Frank or Dick rolled a spliff and we tried to comfort him a bit. "Lucy" actually had the fucking brass to come in the room and ask if she could "talk to him in private." He told her to fuck off out of his house thankfully.