Is that one of them old wives tales?
Them older, spooky females talk like that.
"If ya man comes home from work and he put his work boots under the bed, He's fukking another b#tch"
In your case, I think she wants you to cheat.That way, she'll have some dirt on you, and it'll soothe her conscious a lil bit.Females, ESPECIALLY, can't deal with feeling like "the bad guy" in a relationship.They deal better with coming from the victimized perspective... not being the victimizer.
It's a subliminal suggestion.She's begging you to get your hands dirty.More than likely, she feels like it'll reset the relationship, and make everything even steven.