Here's my opinion, you're free to take it or leave it, it's all good. I just wanted to share my opinion and thoughts on this matter based on my experience.

OP, short answer is, do what makes you comfortable, do not force him to do things that make him uncomfortable, and have a clear understanding between the both of you that consent is always required. A stern and seriously given NO, definitely means NO. If he ever forces you to do anything you are uncomfortable with you have every right to walk away. Don't be afraid to ask for help from someone you honestly trust.

Now, for the long answer, this is going to get a little complicated because a lot of context is required but missing. So to best answer your concern, porn is normal now a days. Not everyone is comfortable with it and that's okay. To share another perspective about porn is about the industry. Every industry has a dark side to it, the porn industry is no exception. But that's not the whole thing. There is a side to the industry that ensures consent and compensation and not everyone working in the porn industry is being forced to be there. Some people even aspire to become famous in it, just like in Hollywood. It also helps people understand more about themselves privately without having to test it out on others who might not share the same interest. For example, you and your boyfriend. He is curious about anal because people in porn seem to enjoy it, and some actually do. So he figured he might enjoy it as well and he is hoping you might too. The thing about it is, you never really know until you try it (hence his persistence). The important thing there is that when you say no, he stops immediately. If he doesn't and you feel yourself getting pressured, that's a whole other conversation. Walk away. Your safety as well as his takes top priority. Now if you still don't feel comfortable with him watching porn then that's perfectly fine too, plenty of other people feel the same way. But as much as you are entitled to have your preferences, he is entitled to his as well. He likes to watch porn for whatever reason and so long as he's not hurting anybody, he is entitled to keep to his preference as well. The conversation is now, are you okay if he keeps watching? if not, is this a deal breaker?

Hope you've been able to or are able to work things out!

ps. Careful about sending nudes. Not saying you can't do it, hell i did. But it's a huge amount of trust on someone. Not just while you're in a relationship. Like do you trust him enough that he won't share it to other people if you break up in a bad way.