Is there a mechanism by which mammals lose interest in a partner after mating?
Well from what I have been taught, and have studied, is that multiple partners for men does not equal more of their DNA persisting, because they are needed to provide for the offspring. So if they impregnate 10 different females and don't stick around, that really doesn't help them pass on their DNA because most would die. So having a family with the same partner does ensure more offspring survive. Humans are picky selecting their mate for the reason of wanting to get the best genetic combination possible to have the most offspring survive. A human child takes a lot of effort to raise to reproductive age, we aren't like some animals who are off and running. So our bodies invest a lot in making sure that happens, being flooded with bonding hormones after sex, women's bodies changing to nurse (and then not being able to hunt/gather/protect themselves) etc. So there is a hormonal cascade that happens to ensure you are going to stick around after mating.

I am really talking about our ancestors, but obviously some of their behavior has carried to us.

However having multiple partners DOES benefit the female. Males can't tell they are the father to a child until much later, after they have supported the pregnant mother, etc. Females can then have their DNA pass on in multiple combinations to ensure survival of more children. Pubic hair in women is thought to have evolved to hide any physical evidence of a recent mating with different males outside of the family. And if current statistics say anything - just as many women cheat on their spouse as men. And when cheated on women want to go after the other woman, because that's our competition and they may be taking the mate we worked so hard to get.

Humans are hard to compare to other animals because we can manipulate so much of what happens to us. But if the ultimate goal is to ensure the passing of DNA then that hypothesis (to me) doesn't fit with humans. 

But then again, I am a woman and therefore could be biased haha. If you read an animal behavior textbook it will go into this in in detail, and state it much better than I can, and with supporting scientific evidence. As a biologist the term "anecdotal evidence" makes me cringe :)