From a genetic point of view I would guess that the majority of men want their female to be monogamous in order to ensure that the children of the union carry their own genes. That one is pretty well hard wired in. Women, on the whole, tend to want a monogamous relationship so that the hunter-gatherer in the family is working on behalf of her children, not disappearing off into the wild blue yonder and leaving her to fend for herself whilst raising the family. It mostly comes down to a question of security for both parties.

Our modern lifestyle may have changed but the genetic hardwiring of both sexes hasn't yet caught up with these changes.

Then you have the role of religion in the mix, with the customs and mores associated with those religions, which compound the faithful unto death idea. Just look at what is happening now with the attitude toward gay marriage. What we hear from the opponents to this idea is that it will have a derogatory effect on family life. The nuclear family unit is seen as the norm and we are generally brought up to believe that that is the status to which we should aspire. It's going to take a lot of time to lose that one.