zorah commented(regarding marriage vs. living together)
Besides, I actually feel more secure knowing my partner could choose to leave anytime, but doesn't. Why would I need to bind him to me?
For what it's worth, I see it as me choosing to bind myself to him, and him choosing to bind himself to me - both of us conceding some of our freedom to leave, rather than either of us gaining power to make the other stay against their will.
Essentially it's an act of faith that that freedom will never need to be used - a gift which says "I truly believe you are someone I can spend my life with - you're not just someone I'm living with until something better comes along."
It's a personal decision, and of course that level of commitment can and does exist without marriage in many many cases - but saying "this person is my husband/wife" is an easy way to let strangers know the level of commitment, in a way that's harder to convey by "partner" or "girlfriend/boyfriend" or whatever.
The original post is here - it's worth a look, raises a lot of interesting questions about the political/social implications of choosing to get married.
On an entirely different note, I've just been listening to the Prisoner of Azkaban on tape again, and I can't help noticing the prominence given to Harry's stomach. It seems to clench, or convulse, or otherwise distort itself at the slightest provocation. Can't help feeling the poor lad must have indigestion ;)