I mean that sleeping with someone while you are seperated, but still married is cheating. It is hurtfull, it destroys the relationship, it makes reconciliation even harder and he never should have done it. A seperation is not a free pass for infedelity. Your husband's view is wrong and he only holds it because it justifies what he has done. He knows it is wrong.
Don't second guess your values. You see things very clearly.