A one night stand
A one-night stand is when you have sex with someone and then you both pretend they never existed. Like a bad friendship.
He said it was a one-night stand. Now he’s my ex and my coworker. That’s the worst.
She had a one-night stand with my brother. Now he thinks I’m a failure.
I had a one-night stand with my friend’s crush. Now I have two exes.