Instead of having a large body of text to explain this to you I'll give you the short version.

EGO.

Open relationships will always end up damaging a person's ego. And 90 percent of time it's going to be the guy. Why?

How do these relationships? Usually someone suggests it (usually the guy from what I experienced) claiming they can make their relationship better and exciting. The women reluctantly agreed. The relationship gets exciting for the first year being generous.

Then...

The guy wants to close it off again. Why? Because he bit off more than he can chew. The woman is getting all the attention, getting sex no problem and the guy is struggling. After tons of failed opportunities to get matches on Tinder and failed dates it will discourage the man to no end. And the woman is just getting started.

So they never work because the guys at the end of the day are always gonna feel like shit because they cant get the same ease of getting partners.

Take a trip to the non monogamy sub reddit if you dont believe me. The posts are all the same