why does your 'other' want to get married? is he afraid of losing you?
why isn't he satisfied to go on this way? do you have more money than he does? LOL! do you have a better job than he does? does he love you so much he can't be without you? ask him. and tell him why you're afraid. he may be able to assure you that it will be okay.
is your freedom more important than he is? if you had to choose between the two, which would you choose right now? if push comes to shove, that's the choice to make.