Let's see, worse than cheating..
Being a russian spy. No one ever believes the spouse didn't know.
Being a serial killer. Don't want to breed with one of those, their genes should not continue.
Gambling away the family home, savings, retirement and leaving the spouse in eternal debt. That's the worst one so far!
Sports fan.
But it's not *just* "worse than cheating", it's worse than cheating but the partner just "shrugs [it] off". I can't really see the partner just shrugging off any of those four.
Well, OK, maybe one of the four. Assuming the Russians pay well, that is.