Men can often have trouble expressing why they're not in the mood...
I know that when I was bigger (or 50 lbs. ago), I still had sex at the same frequency as I do now with my BF. Since I've lost weight, he says he appreciates my body more (as in, he finds it more visually appealing) but that he never found me ugly, or un-sexy, and never used it as an excuse to not have sex, even if he may have hoped I'd lose some pounds.
If it truly is because you've got a little extra weight, then he needs to be able to say that without being insulting or condescending. He needs to be encouraging or it's not going to help anything - just my two cents
It may be due to another reason entirely as almeeker pointed out, but either way you need to be able to have a respectful and open conversation about it.
Female, 30 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 8/30/13 = 179.8 (net: -35.4)
Current mini-goal: 175 lbs.
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130 lbs.
"You don't have to change your life today. You only need to change your day today."