Like Zach, I agree that both sexes are disposed to have multiple partners, however, I have a few points to bring up in response.

Men are roosters. Women are hens They're all chicken. Ever see them get hitched? No. They each have their fun in their own way and then *plop* out comes a fertilized egg now and then. We humans act as if procreation is a goal. IT IS A BYPRODUCT!
An "interesting" argument here. (i.e. I disagree with this) If human children were raised by farmers or cooked for breakfast, surely we would follow this example as well--we would have no further responsibility, after all. However, we do have a responsibility to take care of these "eggs" that *plop* into our laps, and although marriage isn't purely for taking care of the progeny, neither is sex purely for pleasure--each has both pleasure and responsibility. Some would have it all pleasure, some would have it all responsibility, but the fact of the matter is, whether it is fear of children or fear of STDs, you have to be responsible about sex; likewise, even though marriage is a huge commitment, it can also bring great pleasure and happiness. The reason there is so much discord over this subject is easily explained: neither is purely founded on pleasure or responsibility--why else would some choose abstinence and others infidelity?--the hardest part is finding the balance that makes you happy.