I agree that the real issue is between you and your b/f and as long as that has been resolved then THAT mishap can be liad to rest. That said, I've heard it said many times that when you marry someone you are marrying the whole family and whereas I wouldn't be willing to give up my relationship with my b/f because of his family it's just one of the million factors that goes in to the consideration of if he's "the one". It's just that the hurt that his mother caused you will probably resurface many times, but for different, equally lame reasons (she really needs to cut the cord, her baby isn't 8 anymore and she doesn't need to fight his battles, but obviously still wants to or just can't let go.)
I think it sucks really bad the position that his mother has put the two of you in, and like I said it's a bigger deal than nothing, but what ultimately matters is what there is between you and your b/f.