Right click potential spouse. Opens new diplomacy window with marriage proposal to be sent.
Pretty much this. The current set up is that if you left-click, you bring up their character view, if you want to view some more information or so. Right-click to bring up the marriage option in the diplomatic interface. Play around with this and try to get a feel for it. You're free to bring back any feedback you may have for how it works. Which I'll take up with the team if you have any.