When my DH died unexpectedly, I remember going into the solicitor's office to talk about the probate and distinctly remember her saying "well, thank GOD you were married". We'd only got married two years previous, but the implications were enormous. If I hadn't had that old-fashioned piece of paper, I would have been facing homelessness and would have lost a large chunk of my life savings due to the way we're structured the house ownership and our business.
It also meant I received bereavement benefit and was automatically a pension beneficiary. We were only married for two years, but I received those MUCH needed funds and wouldn't have had we been cohabiting for twenty years.
I'm in another relationship now and nowhere near that kind of stage. My boyfriend "doesn't believe" in marriage, which to me is a bit like not believing in a mortgage. You can not agree with it, but its legal implications are undeniable. Having had the experience I've had, I wouldn't make any kind of financial commitment without having it backed up with a legal safety net, whether that's marriage or not. To me, it seems a bit idiotic to have multiple contracts drafted and signed when the one marriage contract does it all, but to be honest I've not looked into it at this stage and I may well find that at my stage of life it benefits me more to stay unmarried.
You can be devoted to each other and not get married. You can marry someone and despise them. I'd only ever want to get married to someone I was devoted to, but the two things don't necessarily go hand in hand. The only certainty and the only fact is that whether you adore each other or not, the legal consequences of marriage still stand, no matter what.
Knowing what I know from being widowed, I don't know that I'd take the risk of, say, buying a house with someone without being married to them. My advice to anyone would be to do what you want, but do it with all the facts. Don't make the mistake of getting swept up in the romance of making a commitment, having a family, buying a house, without also knowing what your legal and financial safety net options are and choosing them carefully.