How do I get my boyfriend to understand he needs to stop sticking his finger in my butt?

I know the title sounds silly but I'm serious. He's asked me if I like it and I tell him no. I have told him I don't like it. It makes me uncomfortable cause it's gross. It's my butt. That's gross. But when we're getting frisky it's like he forgets that I've told him not to do it. My favorite position is doggy so my butt is up in the air and about halfway through there goes his finger in my butt. He clearly enjoys it but I think it's gross and I've told him that but he keeps saying I just need to get used to it. He listens to be about literally everything else so this one thing is so frustrating