I just finished reading Jane Graves’ Tall Tales and Wedding Veils (review to follow), and the heroine partly blames her size 14 figure for her lack of sexual experience. This is very typical among romances I’ve read (well, among the small minority of romances which feature plus sized heroines), and it does make a kind of common sense: if you are overweight (so the thinking may go), you have lower self-esteem, and if you have lower self-esteem, you are less likely to initiate sexual relationships. Or maybe it’s even simpler: if you are heavier, regardless of your self-esteem, you are less likely to be found attractive and thus will have fewer opportunities for sexual activity.
When I was single, I never noticed a correlation between frequency of sexual activity and weight among my peers. Some of my heavier friends were having more sex than anyone else. (I noticed that sometimes the lower the self-esteem, the more sex partners a peer had, actually.)