I would think you would want to be married just for the protection it provides you (Like insurance, pensions health and children issues and I'm sure there are other things as well)
I don't understand why not if your in love enough to live together what is the deal that you don't want to be married.
I myself think that most guys think if they already have the girls living with them why bother with the rest. I think marriage actually protects women the most but I also feel it gives protection to a man as well. Just think what if you had no other family and you became ill do you want the state making decisions for you or someone you love doing it.
And yes I agree marriage does happen in the heart but its always nice to have that piece of paper for protection.
Another example what if your living with someone and say you buy lots of things together and set up a household and one of you decideds they don't like the other and starts cheating and wants out and tells you to get out but you can't have anything because I'm keeping it all. Well in our state its 50/50 so you would not be out on the streets with nothing. Just a little something to think about.
And yes I know when we are in love we think its forever and hopefully it will be but sadly that is not always the case.
Anyway these are just a few of my thoughts on the subject.