Two days later, he turns up at my door. I let him in for coffee thinking he must not have realised it was a one night stand. I was going to let him down gently. He took it alright and after a few tears asks me if we could "fool around" again, without it meaning anything. I told him I wasn't in the mood and, trying to joke it off, told him to have a wank.
The dude takes his clothes off right there, shirt, trousers, boxers, shoes, socks and jerks off in front of me. He gets close and asks for a tissue. I have none. Asks me to pass him a small plastic cup I had on a shelf, which I do, and he comes into that, then throws it in the sink.
I don't know what's worse. That this happened or that I was too taken aback to actually stop him.
TL;DR One night stand came back days later and jerked off in front of me, into a cup.