> It really doesn't. I don't see how sleeping with someone has any impact on the company.

If somebody you have any dealings with does something equally stupid, no matter the context, run. It doesn't matter how much you benefited from the cooperation, when somebody reveals themselves to have such poor judgement of anything, you must be aware that it's only a matter of time before he does somethig even dubmer which will cost you everything you gained so far and more.