I think it might be helpful to try to look at this from another point of view. I myself am divorced. My second divorce was extremely painful and left me with a LOT of healing to do...for a long time, the thought of getting married (and willingly putting myself in the position to go through that again) actually gave me a physical fear reaction. Stomach cramps, clamminess, increased heartrate, etc. I knew I would one day date, I knew I didn't want to be alone for the rest of my life, I knew one day I would probably have the desire to be in a committed, long term relationship, BUT, the idea of ever getting married again was TERRIFYING to me.
I am still on the fence about marriage ever being in my future :) but at least it doesn't bring fear to my heart anymore, and I can have the idea in my head without experiencing a fight/flight reaction, lol...
Having said that, maybe your boyfriend has similar feelings regarding marriage? That would make a lot of sense to me. I've been married twice, and for me, both times, getting married did ruin it, so I see where he's coming from.