My best friend went through this too and their marriage has never been the same (her daughter is now 2 years old). You really have to talk about it and tell him how you feel. If he can't look past your beautiful belly and the magic that is the female body, HE has the problem and might need some counseling.
I have to ask though, how do YOU feel about your body? If you feel beautiful and magical about your amazing body, he might follow that lead. Any weight you gain is all for this growing baby that you both magically created together in a beautiful intimate moment.
When I was pregnant, my sex drive was way up and my husband was usually eager to comply. It was never about my body that he said no, only for being too tired or not in the mood himself. We were creative with positions to accomodate for the baby belly even in the last weeks of pregnancy.
Marriage is about for better or for worse. And unfortunately our society does not celebrate pregnant women as they should be. Pregnancy is definitely one of the better parts of life and marriage. It is a beautiful time of life and your hubby is making it ugly.
I teach prenatal yoga in Brooklyn and a big part of my class is celebrating pregnancy and the new bodies you have. Everytime your baby kicks, you should feel how amazing you are and your body is.
I hope this all helps.