I think you should never cohabit with a man if you are not married. Marriage is some sort of a guarantee - well, in case you divorce. Otherwise you are providing sex on tap, housekeeping services and all the time he is looking for a better option, whilst your reproductive health suffers and he is able to breed into his 80s. Look at the threads on here - oh, we have been together for X years and he took off - what recourse do I have? Bugger all is your recourse. Oh, I helped to build up his career and he has just hopped off into the sunset with a woman he works with.... What recourse do I have? I don't know why women are so intent to make the same mistake again and again.
Obviously, if you are playing the long game (Kate Middleton) and what is her face that snatched the Duke of whatever when the Duke was on a 'temporary break' with his long-term girl-friend, other rules apply, I am sure.
But if they are not oligarchs, or royalty and you have no 'cunning plans for future', why let those wankers get away with no obligations?
Obviously, if you are the Duchess of whatever/Senior Partner/etc. and he is a plumber, then you may not want to marry him, but in the majority of cases it is bog-standard people playing stupid games with non-committing.