but it's also not an unreasonable boundary in our relationship

This is extremely subjective. Just because you think it's not unreasonable doesn't mean your bf thinks it's reasonable.

He once got a bit offended when I told him I found one of my TAs really hot, and we've both agreed about being strictly monogamous and not interested in threesomes/swaps/etc.

I mean this is clearly different. This is a person you know from real life, someone you had regular contact with. And just because you mention you think someone's hot doesn't mean you don't want to be monogamous.... but yeah, I see that you might interpret this as a kind of double standard.

The parts that are particularly distressing me are that a) these are girls he knows in a non-sexual way, so they aren't "abstract" the way porn stars are, and b) I have brought up the issue before ("I don't like knowing you see nudes on twitter and instagram") and he has continued doing it.

So he knows this girls from school etc? Then it clearly is a double standard.

It's really hard having to admit to sneaky behaviour in order to get there.

Tbh, I don't think this relationship will work out. You're clearly not compatible. Not only have you got different views, your bf doesn't also see an issue with double standars. From what you've told us, he won't change.