Monogamy is like being duct-taped inside a full Barney costume. Maybe you put in on because you wanted to, and the costume is cozy and warm in the winter months, but then the summer comes and suddenly you're so hot, and you miss your peripheral vision, and GAGGGGHHHH, get me out of this fucking goddamn Barney!
What I am trying to say here is that sometimes people cheat on each other, and, according to a study done in collaboration by the University of Stirling and Benedict Jones from the University of Glasgow, when men are searching for a quick and easy fling, they look for women with particularly "feminine" features like a small jaw and a full face. The researchers came to this conclusion after they asked several hundred straight men to rate composites of women's faces in terms of long-term or short-term relationships. Not only did the men rate the feminine ones higher for a fling, but men with already in steady relationships were even more likely to do so.
On the other hand, if you have a jaw like cut glass and cheekbones like David Bowie, you are likely to be the other half of that steady relationship — men choose "less attractive/less feminine" women for long-term mates, and the researchers mention that women do the same (but less attractive/less masculine).