Why do we live in a society that blames a person who cheats in their relation, but doesn't get on the partner who caused the other to cheat?

If a guy is caught cheating with another girl, that means his girlfriend wasn't providing him with something... so he looked elsewhere for it. Why is this seen as worse than the girlfriend not fulfilling the needs of her man? If the girlfriend was sufficient in taking care of her man, he wouldn't need to be satisfied by someone else.

(Can swap the genders in the above post, and still would hold true.)
Why do we live in a society that blames a person who cheats in their relation, but doesn't get on the partner who caused the other to cheat?
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