boyfriends butt obsession

So my bf and I had been dating for a few months before we started discussing trying anal. I had done it before with my ex but he said he had never had a girl allow him. So we did it and ever since then it’s all he wants. Every once in a while I think it’s nice bc it can be a fun change but I don’t prefer to do it every time we have sex. It’s also really hard for me to enjoy it with him bc he is rather “bigger” down there. It’s to the point now when I try to have normal sex he turns me down for whatever reason and it makes me so upset. But if I were to offer him anal he would be down to do that in a heart beat. It’s not like I have a nice ass either. Due to a lot of stress I’ve been going through this past year I have lost an extreme amount of weight so my cute little bubble butt is now gone and just kind of flat. It makes me wonder if I’m like “loose” ever since I had my son and now anal is all he can enjoy with me or maybe something else worse is going on. Has anyone ever experienced this with their bf?? Help!!!