There's a certain degree of blame to the cheatee's side, but it depends on the situation as to how far that blame applies. I'd say the blame lies 50/50 with both if say, the person is a friend of the partner who is being cheated on and actively goads the cheater into the affair. The blame lies solely with the cheater if it was say, a drunken one night stand on the cheatee's part and they feel unaware of the circumstances.
Intentionally sleeping with those who you know are in relationships is wrong, because you are knowingly putting someone's mental health and happiness and risk for a trivial and selfish thing.