This is terrible!! I can't imagine anyone doing to me, especially my husband who met me at 185 and married me at 207 and watched get to 252 (pregnant) and then remain at 227 until I got serious at losing weight and he NEVER said a word or made me feel bad or guilty. Just by side loving me. He is supportive of most things I choose to do (except maybe spending money!).
The only time people don't listen when I say no is at work for birthdays with that

cake being offered over and over.
Oh and my mother in law who brought me 2 DOZEN doughnuts the day after I told her about my diet. Maybe coincidence?

She is overweight also and I think it's the same thing you are talking about with your boyfriend, a combo of guilt and maybe even fear of being "left behind"