I definitely think there are a large degree of variation between people on the subject, but that on average (in general) biologically we only really pair for about 3 years, after that the pair bonding tanks quickly and the amount of work required to maintain an exclusive relationship increases exponentially (with a cap somewhere near the 10 year mark). I also think that we can overcome our biology, either way.
I also don't think that pair bonding needs to be exclusive, I think we can pair bond with multiple people at the same time. This might (probably is) variable for different people, and different combinations of people. So that some people can only form single pair bonds together, but separately those same people could with a different group form multiple pair bonds, some people will only be able to form a single pair bond with anyone, and some people can barely pair bond at all.
I think if we could find a way to test this we would find a bell curve rating how easily people form pair bonds where most people can form multiple pair bonds simultaneously, but they have trouble maintaining any pair bond more than 7 years, with the average being 3. The people that can only ever form a single pair bond at a time or can't form any at all and those that can form dozens are outliers on the far ends of the bell curve.
I think that most people in our society work hard at not forming multiple pair bonds, and I think that it shows pretty clearly in our culture.