this i think is avery interesting question! i would actually say myself that monogamy is biological, my reasons for this are not religious as such but more to the point of natural respect! a swan for example will stay with its partner for eternity, so like many other species in the animal kingdom. i dont think i can agree with Helen Croydon though, she must have a heart of steel! in many cultures you can under the "religious authority" have more than one wife, but in saying that after speaking to some friends who come from countries where this is allowed, they told me it causes more grief than tranquility!! anyone who is anyone thats had experiences of being "twotimed" will agree the heartbreak and distruction the said affair can cause, along with the misery of mistrust. again having more than one partner can in itself bring harm due to sexually transmitted illnesses. i wouldnt say i was old fashioned, but realistic. my view here is that if you and your partner are open to discuss problems and steer the relationship in a "mutual" capacity, working as to what you both like and want. there would be no reason to find another to fill the gap that causes polygamy. life seems more serene when two work together than having many, after that it gets complicated and heartbreaking!
thanks for posing question.