Your story reminds me so much of my own. I, too, did not date much. When I realized that my now ex was going to propose, I had very similar reservations about marrying him. After the divorce was finalized, I became very honest with myself....the main reason I married him--inertia.
I felt like all the momentum was moving in that direction, that I did not want to make a scene or rock the boat, that I would feel too guilty hurting him if he cared enough about me to ask me to marry him. And so I did marry him, in spite of my reservations. The week before the wedding, I had a horrible, nightmarish dream about he and I that I think was my subconcious throwing out a last ditch effort to stop the wedding, but I went ahead anyway.
If you really have deep-down reservations about your boyfriend, please heed them. Please don't spend your 20's and 30's the way that I did, having a marriage that was not with an equal partner but with an immature "teenager" who I felt I had to "mother".