We already got this with the ceremony of eternal bonding.

You can't have both, how would it make sense for me, someone who is in-game married to my IRL partner, to go on a romance mission?

The most you could hope for is a Skyrim-esque marriage, where you can marry some NPC and have a wedding with them if you have no player to marry, then they can move in your house. But that would just be a waste of developer time imo.