Sorry but I agree with your boyfriend when he says He said he shouldn't be made to feel like he has something to apologise for to someone he didn't even know then
It's the equivalent of a new boyfriend trawling through all two thousand photos of her on Facebook and then kicking off about behaviour he doesn't like that she did before she even met him, maybe kicking off cos she once posed with an Abercrombie male model in the store, or she put bikini photos up, or photos of her hammered, whatever it is. We would rightly be saying massive red flags, run from this guy!
The fact you went to the effort of trawling through all two thousand of his followers to check individually if they were glamour accounts is concerning. A normal response would have been to notice a few then speak to him. Ask whether he keeps up with their posts and what made him add them, and when. To get an accurate picture. Around 1.5% of his follows are these accounts!
It's fine for you to decide you don't want to be with a man who follows or has ever followed adult accounts on twitter, absolutely. Draw that boundary and walk away, you have every right to. But he is absolutely right to be annoyed and disappointed that his new gf of a few months is unable to approach him about something calmly like an adult and instead gets upset, invests the time it would take to assess every single one of his followers, and then sit and drink because she's so hurt! He has nothing to apologise for and you've fucked up the chance to sort out your first slight disagreement/difference of opinion/difficult conversation being so melodramatic. If I were him I'd be reconsidering the relationship, he's right to assert himself and not go straight into trying to placate you and apologise (for what? He's done nothing wrong. It's just a difference in what you both find acceptable but neither is wrong).
If it'd happened to me I'd probably have teased him about it (and how hope he likes that all his friends can see who he wanks to) and let it go. If he added them while with me I'd find that disrespectful. But before he met me? What's wrong with that? And it's not usual to get into a new relationship then suddenly start assessing all of your social media history to ensure there's nothing dubious on there. If he'd followed a few porn stars no big deal, he probably liked some videos he saw them in. He could access their twitter whether he followed them or not so not sure what him deleting it all would achieve.
But I'm secure in myself, we both watch porn, I'm happy with how I look mostly and secure that he fancies the pants off me and vice versa. So it's no threat. I think you were spot on earlier that you need to work on your low self esteem as that's the key to why this has bothered you so much and why you've handled it this poorly imo. And it's so early for you two. That's on you to improve and work on, not him.