Scientifically speaking, males lose their sexual drive and attraction towards a partner they have been with for a long period of time. So when you have two men in a relationship, this happens on both ends. If sex is important part of life for both, then they will experience sexual frustration which can be resolved by either ending the relationship or opening it up. Worst case scenario is that it ends in cheating and lying.
Culturally speaking, for an advanced society we are still holding on to some pretty ancient sexual dogmas. We are still pretty sexually suppressed, which is why advertising that uses sex is very effective. It is easy to sell a fantasy to people when they aren't willing to get it in real life.
Historical research shows that even for straight couples, monogamy is not the norm. It is just a lot messier than it is for gay guys. Between infidelity, divorce, and serial monogamy (people who jump from one short term relationship to the next) monogamy is not really the norm. I think it is just easier for people to do what is socially acceptable to get more sexual partners than to just have an authentic and honest conversation with the person they are with.
Most relationships I observe are like a hostage situation. Where one person demands an exclusive lease on the other person's body and thoughts, and they threaten abandonment in case the other person does anything to escape that.
People just have to be authentic and communicate without fear, when that happens you will see a lot more people having open relationships.