Wanting monogamy and being sexually satisfied with it are two different things. There are a lot of social pressures and demands in play - mostly for women, but for men as well. And, the research on this - from huge studies from around the world - is that women often lose their libido 1-1.5 years into a committed live-in relationships, not because women are less sexual than men are, but because they are bored with the same old rote and routine sex or otherwise desexualized due to roles (as house elf) and over-familiarity. Most of them don't want other partners; they want more interesting, exciting sex -although 2/3s of mongogamous relationships that open up, do so at the suggestion of the woman, so there’s that as well.

If You Want Your Wife to Be Your Lover