I don’t think people are biologically programmed to be with only one person for their entire lives.
Countless studies have shown that men have a natural biological urge to mate with as many women as possible to ensure their genes live on long after they’re gone. Studies on women have shown they have a natural tendancy to choose the man who will be most able to provide for her and her children.
Personally, I think marriage should be a contract: a 2-year, 5-year, or 10-year contract, depending on the wishes of the participants. Then, if at the end of the contract the couple still wishes to be together, they can renegotiate their contract.