in DR, MWD doesn't claim that all marriage should be saved. and she notes that marriages where there is abuse, addiction, or chronic infidelity should not be saved.
so you are right. people with spouses who cheat probably shouldn't try to save their marriages. once a cheater, always a cheater. even if they do it once, they are likely to do it again.
i couldn't handle it if my h put his thing in someone else. that would be a dealbreaker for me. i know i deserve better than that.
