It could also be because it is the moral choice of lifestyle.
Remember, sex involves powerful feelings both biological and emotional. It is only recently that we have felt the need to separate them out, and many believe that this is detrimental to society. If I give my love to someone, emotionally as well as physically, I would very much like to know that they would wish to do the same with me, and me alone. Regardless of how cosmopolitan the current fashions in society seem to be, we still feel very much betrayed when we discover that our partner is having sex with someone else, not to mention the cold feeling when they shrug it off as unimportant.
Long lasting relationships rely on loyalty and trust, and grow towards exclusivity because the couple become independent of others for emotional support. Concious choice or not, it is better to be monogomous, and this behaviour is perhaps built into us genetically as it is likely to lead to a greater chance of successful offspring, than the alternatives.
In response to synx's enlightened comments I would like to submit the following:
while the two may not be exactly the same, in most cases the ethical choice IS the culturally acceptable choice. Were this not the case, we would be living in hell. When it is proven that the culturally acceptable choice IS NOT ethical, then it inevitably follows that reasonable people try to change the culture to become more ethical.
Monogamy is a case in point.
It has survived so long, and spread so wide simply because it represents the keystone to a stable family environment that all parties would agree is mutually beneficial. Only in recent decades has the family model been under fire from highly individualistic trends from society, I believe this will pass. Families are necessary for society.
Jealousy isn't conditioned, it is natural. Ask any child, or child psychologist.
I didn't say it was better to be emotionally dependent on one, and not many. I IMPLIED (and still agree with the fact) that it is far better to be emotionally dependent on a *stable* one than a fluctuating many. As it is highly unlikely that you are able to command and give the same level of love and devotion from and to five partners than if you only have one. This isn't a radical new social theory. It's just common sense. Or failing that, maths.
As for the final note, fair enough, but WHY BOTHER? If the natural alternative is better (see above for proof) then why take the stupid option and be wreckless with not only your own heart, but with lots of others as well?