What would you do if you had a one night stand with someone and didn't find out they had a girlfriend until after you had sex?

Never have anything to do with him again. It was a one night stand; leave it there. If you did anything more with him it would be immoral and stupid. He would do the same thing if he dated you. It was a one night stand so what are you feeling guilty about don't worry about it was a one night stand so obviously he didn't mean much to you because you slept with him the first night so don't worry about it go on and forget about him