Ever had sex with someone that you shouldn't have? You know that, realistically, that person should be off-limits, yet you bang them anyways?
Maybe you cheated on someone with a random person, or you were the other woman/man to a married person. Both scenarios are horrible, and you know what you did was wrong.
People have relationships with people they shouldn't all the time. They know what they're doing is wrong, and often, many people feel no guilt at all.
And it's not just cheating scenarios that are wrong. People sleep with step-siblings, cousins, employers, and many more people no one in their right mind should sleep with. So if you're contemplating sleeping with someone you're not sure about, listen to your intuition, 'cause that sh*t is always right.