Whilst ... either of those choices seems the logical solution, one ... ... or even BOTH of them could be weighing~up his/her/ their options - regarding the legal implications arising from say, property disputes, &c. - should they later decide to split.
would be my guess, anyway.
You don't have to be married or living in a civil relationship or any other way of putting it.
If you are a couple and problems arise you are done for and it is in my experience the bloke get clobbered,