Discovery News has another reason to diet and exercise: being fat hurts your sex life.
Apparently this comes from a statistical study. The higher the participants BMI (Body Mass Index, which admittedly is just a mathematical measure of weight vs. height rather than an actual measurement of how fat a person is), the less often they had sex, the more often they had sex-related problems like impotence, and the more likely a woman was to mess up on birth control and wind up with an unplanned pregnancy. (Men apparently were not impeded from condom use, if it went up at all in the first place.) Also, the obese were more likely to contract an STD.
Part of this is likely sociological. We live in a society that rules that being fat is gross. But part of this is also medical. Being overweight apparently is hard on a lot of your body's systems. If your body feels it has to sacrifice something, sex is often it's first choice. (After all, if you're not healthy enough to have sex properly, how will you manage the possibility of parenthood?) The two likely combine with each other, where fat people become more insecure, and insecure people do less to avoid being fat. And people wind up both and desperately unhappy.
I think the evidence suggests that obese people are being more sloppy about sex when they do manage to have it. Possibly because it's so rare that, well, can't be bothered with abstractions like STD and pregnancy avoidance.
Next, the study where we figure out something useful to do with this.