The term Common Law Wife is often bandied about but in legal terms there is no such thing, well at least not in England. You are ether legally married, whether it be a civil partnership between male and female or both of the same sex or you are cohabiting when you have very few rights or legal protection. People who are cohabiting can obviously draw up legal agreements concerning financial and inheritance matters which can help but probably won't work under AFPS rules which don't, as far as I know take into account cohabiting couples
I know many unmarried couples that have been in a very loving and stable relationships for many years. Indeed my present neighbours are both in their 80s and have been together for over 20 years and good luck to them and they have taken professional advice regarding of disposal of assets when one of them dies. However how do you define a common law wife? Is it someone you picked up at the NAAFI bop last Saturday night and you have been shacked up with for the past week? Is it someone you have been living with for X number of months/years? Is there a time limit? Is it a case of you walking into SHQ and saying - person X is now my common law partner and gets my pension? Can you change this on a weekly/monthly basis depending on who you pick up at the NAAFI bop? Being a bit silly there but you can't easily define and regulate it. The easy answer is of course a legal marriage but I know that is not available to some people who for whatever reason can't get married.
Many civilian companies allow you to nominate someone to receive your pension when you die so perhaps this is the way forward for the military.