Cheaters lack so many qualities that quite frankly, I don't understand why anyone would want to fathom their reasons for cheating because therein madness lies, except to say they feel entitled to shit all over their spouse and if they think along the same lines as you OP, prefer to place at least some of the blame on the true victim of their self centredness.
Your theory OP sounds bonkers and isn't rooted in reality.
Incredibly fit and beautiful women get cheated on every day of the week.
Enormously rich, successful gorgeous men get cheated on.
It's almost like you're trying to put some of the responsibility for the Infidelity at the wronged person's door!
You sound like a cheater apologist!
I suspect you are an OW and have started this thread to reassure yourself that what you believe is true, which is that wives who 'let themselves go' are partly responsible for their husbands dicks going awry.
The truth is that people who cheat can dress it up in whichever pretty outfit they like, but in every single case I've personally ever known of, there was never ever any intention to leave their spouses, only to have their cake and eat it.
When they are rumbled and their spouse boots them out, they go running to the OW/OM because they have no other option but they still beg their ex to take them back.
Even if their spouse had 'let themselves go', they're still an awful person to cheat on their spouse and even if that was the real reason, which I doubt, they are demonstrating that they'd rather live with someone they find unattractive, lie, conceal their actions and cheat, than have a conversation about their waning libido and that sounds like a shit partner to me, someone who chooses to shit on their partner rather than speak to them.
If they have such little respect for their partner, they could just leave?
Cheating is disrespectful and unnecessary. It just requires the cheater to make a bloody choice before they cheat rather than believe they can have it all.
The only reason cheaters lie and sneak around behind their spouses backs is because they don't want to make a choice and they don't want to be honest with their spouse because that may provoke their spouse into making a choice that wouldn't benefit the cheater.