If people cohabiting were to get the same rights as married couple when does this apply?!
You could meet someone and 3 months later decide that they were "The One" and then:
A) marry them and then you would instantly have all all the legal protection
B) not marry them and have to live with the consequences of not having the legal protection
If the dreamland some people want to live in how long would you have to be together to get legal protection? 1 year, 5 years, 10 years, 30years? What if something happened just short of the deadline?
It would also be unfair on people to just lump them with the extra responsibilities without them choosing to do so. "Dear Bob, you've been with Jane for 5 years now so all your assets are jointly owned. Good luck." what if Bob and Jane weren't sure they wanted that commitment, what if they were in a rocky patch and were about to break up?
Oh and when the hell do you count from?? With marriages there is a very clear date, when you are dating its usually more of a gradual thing.
Do you actually have to be living together? What about people who can't do that for a very valid reason? What about long term relationships?
The only way you should get the legal protections and responsibilities is but consenting to it and signing the damn piece of paper.
And I say all of this as someone who is currently 8w+5d pregnant by her boyfriend of 3months! (Yes I know it's ridiculous, no I don't plan to stop working and will be saving like mad and making a will because I am in a more precarious position than I'd like.)