I have mixed feelings about the open relationship.
If done right, I think this type of plot would be great on the condition that the MC is an attractive alpha male. I'd imagine the female partner having dates with some guys and then comes to the realization of just how far above the rest her true partner actually is. I would love to see scenes like that honestly.
Sadly, the MC of Sunshine Love is not one and this makes it depressing for me. His situation monetarily and work-wise is worse than Nicole while his sexual prowess is far from satisfying (I mean, their first sex scene had Nicole needing to fake orgasm, and the flashback sex scene actually had MC fell asleep before Nicole got off. That is so not cool!). This set the situation where it's easy to imagine Nicole to end up with someone better on both end of what the MC lacks because Nicole is a very attractive woman who also had the brain and the work discipline.
Honestly, if I view it as a third party instead of inserting myself into MC, Nicole definitely deserves someone better.