Well I am well over 50 yo myself by now and have a bit more as just a few examples to sample from due to my life and work. Thing is though long term relations are about emotional needs being fulfilled for both parties and those hanging around a couple or throuple or even more people in a give and take situation for all parties involved.
When for one or more of the parties in a relation those needs are not fulfilled long term or there are always tensions since main people in the relation and hanging around it and made important have opposite needs and goals, those relations stop functioning over time whatever the nature or the deals you made how open or closed that relationship will be. It has nothing to do with open relationships as such, but with all relationships, open, gay, lesbian, straight and so on and their nature. It goes even for just basic friendships, just look at the ones you have had over years and years and still work 20 years later even if those had their ups and downs as well and might have been closer or less close over years depending how easily you could integrate them with your main relationship.
That does not mean people will not stay together for important reasons like the kids. mortgages or just simple fear of being alone long after the relation has gone south, but those are also the relations I know where one or both of the two people is a serial cheater since they cannot resist anyone that does fulfill those emotional needs for them and give them attention or obsessed with their work or playing computer games in a separate room all day when not at work 7 days a week.
Well trusting your wife or GF when someone else flirts with her in a not open relationship, if you cannot do that, why would you be in a relation with her anyway in the first place. Trust is one of the foundations of any relationship and if that is not there, there is no relationship to start with ......
