Listen to me very carefully because I used to be a controlling, jealous, insecure boyfriend.
He is WAY out of line if he thinks it is not OK for you to get a massage for your injured back. You are not being unreasonable at all. His point of view is unreasonable. I do understand what he is feeling because I used to get upset at stupid things like this too. Here is what he sees. "Oh no, my girlfriend is going to have some hot, sexy massage therapist guy giving her a massage. He will probably see her half naked and she might get like him better than me and cheat on me"
I'm guessing he doesn't like you going out anywhere without him too right?
He feels threatened by this for whatever irrational reason he has. Most likely, he is insecure about himself, and doesn't think he is good enough for you in some way. He needs to realize a few things.
1. He needs to learn how to trust you. He can't watch over you 24/7. You will be out somewhere and you will have the opportunity to cheat on him. If he really cares about you, he will understand and accept this because he knows that you wouldn't do anything to hurt him. Him getting upset about a massage shows a major lack of trust and maturity on his part.
2. He needs to accept the fact that you choose to be with him. You are not forced to be with him. You choose to because you think he is the best fit. Once he truly understands this, he won't worry about other guys.
3. He can't think from your perspective. If it was his back that was hurt, I'm sure he would have no problem getting a massage from a girl to feel better. He is being selfish and he is scared that your actions in some way mean that you don't care about him as much. The fact that he doesn't offer to give you a massage really shows how immature he is. If he really wanted you to not get one professionally, the least he could do is offer to give you one.
Conclusion: He is the insecure, jealous, controlling type. He is dependent on you. He feels like without you in his life, he won't ever be happy. That is why he gets so protective over you. Sadly, it is hard to get through to a guy like this about their habits. My girlfriend told me all this and I didn't believe her. It took one of my guy friends setting me straight while we smoked weed together before I understood all of this.
Good luck.