I know many people who were and are in open relationships. Here is the thing. You are supposed to be spending most of your time with your partner, strengthening those bonds. But say you go out every other night with Person Y and are having sex with them. You don't just climb into bed, have sex, and then leave. You hang out, drink wine, talk, snuggle. All of those "bond creating" activities. If you're having problems with your current partner, Person Y becomes a shoulder to lean on.
So it is extremely hard for that NOT to create a wedge in the primary relationship.
Look at the song "Layla".
