I get you. But I think, honestly, most cheaters are into the feelings and sensations they get from cheating, much like a heroin addicted person is into the high. An addict likely won't care if they have A+ grade heroin at home, if someone offers them heroin at a party or a dinner, they'll gladly accept it because why not? Whether they have the bestest stuff somewhere already or not, they won't refuse that extra chance to get high.
That's why it's a matter of character, because that's where loyalty comes from. A person who seeks or takes that chance and not care about the commitment they have with someone else is person without loyalty - and that's ultimately a flaw in their character, not a flaw about their partner.