You know, it wasn't all that long ago when I was saying the same things as you are now, DSF. In fact, when I first read your post, I was startled at my own perception that you were writing defensively... but then I reconsidered that *I* might have been feeling defensive when I was in a committed relationship but not married, so I realize that I have to be careful not to read that into your posts, since you are, obviously, not me
You may very well feel no different after you get married. If you look at it as just a piece of paper, then you're right -- it really doesn't matter whether you get married or not. However, if you and your partner mindfully and meaningfully decide to take that step together, marriage can mean much, much more than just a piece of paper. I don't believe that any relationship is 'forever' just because someone gets married, but I just know that it added an entirely new dimension to what was already a fulfilling, deeply committed relationship for both me and my husband.
What I mean is that you'll get out of it what you put into it.