Oh - about the comment earlier that got so many people's dander up. I see both sides of that coin; being heavy doesn't mean you don't take care of yourself!
BUT - I know plenty of people who got comfortable in a relationship and "let themselves go", caring less about their appearence and gaining weight, people who used to keep in excellent shape. And many of these people complain about their sex life diminishing, which is just inviting the comments from people without tact.
The dead givaway is when they become single again and drop the weight in a couple of months, buy new clothes, and start maintaining their appearance again. Tsk tsk.
Point being, it CAN be the case, it's just not ALWAYS the case.