Oral sex pressure - husband 32M and wife 31F

My husband 32M is pushing me 31F for a bj, fine, not my favorite but I’ll do it. Our 10 year wedding anniversary is this week and he’s asking if he can cu* in my mouth. I said no, absolutely not. We’ve tried it twice, and both times ended in my crying, gagging and nauseous. It absolutely disgusts me, I hate the texture, the taste, everything. I don’t feel he should be trying to pressure me just because he likes it. I don’t understand why he can’t just be happy that I’m offering a bj?! I’m not sure what to do because he doesn’t seem to understand me and it’s an argument that comes up constantly and I’m tired of it.