Well,
ryansFLYIN is a condescending prick about it, but he got the gist of it right.
If you have an SO, you'd want them to have the freedom to do their own thing and come back to you out of love, without getting involved with someone else while they're at it.
Do you really need an SO that you have to chain up in the basement or police their every move just to prevent them from cheating? That only shows that their loyalty was never genuine in the first place.
John Marston from "Red Dead Redemption 1" is loyal to his wife. He can be approached by prostitutes, and he has 9 different responses, all of which come down to "Sorry, ma'am, I'm married!" He doesn't need Abigail to be there, holding him back from cheating.
If an LI cheats on the MC just because you didn't intervene, she's trash; it's just that simple.