No, for one thing, you absolutely don’t need an excuse to not be comfortable sending nudes. It’s something you’re allowed to just be inherently uncomfortable with. Secondly, you’re VERY much in the right. If he thinks a breakup is truly unthinkable to the point where there’s no reason to be careful about a potential future where you aren’t together; ask him why he hasn’t proposed to you yet
I understand why he would take it like that. This probably just means he’s insecure which is completely normal. Just make it very clear that 1 you are with him and you don’t have any intentions of breaking up (assuming that’s true) and 2 say you just aren’t comfortable with nudes yet bc u never know what could happen but you will surly let him know when you are ready 😉