I don't see the point in marriage at all here.
Firstly, you're in an open relationship, (the opposite of marriage) then still add living separately into the mix (unless he inherits a house though) and what we have is NO ACTUAL REASON to be married.
Why not just stay together without the formalities, its much more logical. Doesn't mean you love each other any less, plenty people don't get married yet are committed to each other.
I am probably one such person, although I'm at least open to the idea of marriage now.
For a more obvious couple, take Oprah and Steadman for instance...
You asked if its weird, the answer is plain and simple yes, the other half of my answer is that if I were to bear witness to such a union I would think the marriage a complete joke.
But maybe that's just me, although I highly doubt that....