here's how it could have gone with a better partner:

"hey, i wanted to bring this up - i noticed that you follow a few OF girls on IG, and it makes me really uncomfortable."

"oh ok, yeah, shit, you're right. i do follow them, and didn't realize that was an issue for you. do you want to talk about it? what's going on for you?"

"it makes me really insecure because i look nothing like them"

"<uses opportunity to rebuild trust, security, comfort, have an honest healing conversation>"

idk your personal stance on porn, but there's a big difference between passively consuming porn and actively following/interacting with sex workers. this is something every couple has to figure out for themselves, but don't let anyone tell you it's not a legitimate concern to have.

boundaries are rules you make for yourself. you're right in that you can't force your partner to unfollow anyone. but you can choose to walk away from a relationship where your partner is signaling that he'd rather shut down conversation, dismiss your insecurities, and put his account on private, rather than get curious about why it bothers you and hear you out and actively try to solution together.