If a person has sex with someone they know is in a relationship, are they just as bad as the cheater? How 'bad' are they?

I assume they’re not a bad person if they were lied to, but if they already know the person is married or in a relationship, how messed up is that?

I just used the word “bad” because I didn’t know what else to say. I don’t think they’re actually a horrible person 100%. And this isn’t about me either. I was actually thinking about websites that promote it like Ashley Madison.