I agree with what you are saying. I do think what he is kinda being weird about it since he never set any boundaries. So I guess I'll retract my statement.

However, I don't think that he's controlling, or emotionally immature. I can see why a guy, on the surface won't have a problem with it. But then after the action being done, be uncomfortable with the fact that she posed nude.

even if being in a monogamous relationship automatically meant you were uncomfortable with your partner posing for an artist, which is not necessarily true, since that's not really a sexual thing.

It means a straight person of the opposite sex, gets to see your partner in an intimate light, in the nude. It isn't just posing. It's posing in the nude. It might not be a sexual act, but it is still somewhat sexual as nude art is usually erotic in some way. If she modeled on the regular, than yes, that would be something the boyfriend would have to get used to. However, based on the post, she was not a regular model. So I can understand why someone would be uncomfortable.