Yeah, no one beats Marge with her hair down!

But let me add two more things:
1) With her weird hairdo kept by Jenson's water seal Marge can be sexy, too. What about her "lovey-dovey" look? The way how she looks on Homer, oh, that's incredible, how they operate with her face expression.
2) I really don't think Marge was made attractive just for fanservice, for something, as Marco said, "catchy".
Yep, they made some exaggeration enlargening Marge's... uhm... women parts, but in other cases it's part of her unique character. I admire very much her ability to stay tidy, beautiful, attractive for so many years, even with such man as Homer. She could become lazy ugly fat chick, like her sisters (in one episode they went to barbershop together, and oh boy, Patty and Selma aren't so ugly, they just don't bother very much about themselves). But she loves Homer, she respects herself, she is a grownup healthy woman and her sexual interest is an integral part of her.
LeifNice grabs! She is very beautiful.
runfromcheneyAbout situation you've mentioned with Marge being too fat, I was impressed more not with that moment, when she compared herself with the cardboard woman, but the one in "Large Marge", when Manjula grabs a big fold on Marge's belly

But still, I don't think Marge's figure and her ability to be sex appealing are related that much. Plump women can be attractive, too. But it's important, that she likes her usual figure and she cares. She'd say "Oh, what's the point. I'll start a diet someday, later", but no, she really cares about herself, on a daily basis.