I saw a documentary type of show on Discovery, I believe, and they were talking about the differences and similiarities in sexuality between human beings and other mammals.. Something that I found interesting is that supposedly like other mammlas, it's in our nature to want to have multiple sex partners..
Animals go around and mate with another animal, but then they move on to another one.. and another one.
They were saying that the idea of spending the rest of your life with one partner is going against how we are "wired." And supposedly, that's why it's so difficult to stay with only one person.
Obviously, it's possible, though.

I think women do this maybe because they aren't getting what they want sexually from their partner and so they try to find someone else to fulfill their needs. It's the same with men.. that or they get bored with their partner and want some variety.
maybe?