As long as they are happy, it shouldn't matter. Shame on those who use religion to judge. Stay away from organized religions, they are nothing but bad news unless you just want social support and networking.
"Organized religion has been giving people reasons to judge and hate others for quite a while now. (not to mention being a key factor for MOST of the misery on our planet - past and present THINK ABOUT IT)" X 10!!!!!!
Was married 15yrs, lived together for about a year before, dated 2 before that. Should have known better, I was young and dumb. Am paying for it with alimony now and very bitter about supporting a lazy slug. Something else your cousin should pay attention to is that he has lived with her for so long it is basically a "common law marraige", so if they ever separate, there is a standard of living, joint assets etc. Not that they will go down that road and it sounds like they are happy and committed so it shouldn't matter. But if they didn't get married, methinks there is a small part of them that says,"someday it just might not work out."
One possible benefit from marraige as I am now finding out is financial rewards that have been institutionalized from insurance breaks to tax breaks, additional pays (in military there is a family separation allowance). I am with someone now, but will never marry again. As a result, it costs somewhat financially.
Good luck to them and you.