because that aint the main reason people cheat or shyt falls apart. people cheat with much less attractive partners, people cheat with folks who are fatter or the same size as their mate, most cases, cheating aint no where near about one person in the relationship not keeping themselves up, it's either because they're missing something in the relationship, they had an opportunity, or they're just trifling
and if you feel someone is disrespecting the relationship by not keeping themselves up, wouldn't walking away/breaking up be the proper thing to do as opposed to cheating