I'd be all for marriage being an option as something discussed in dialogue and completed post-game, off-camera in an ending slide. As in, you can discuss it and decide whether or not to marry, but then resources are not sunk into having a wedding scene play out--particularly during the heat of the plot's actions.
Not that I'd be opposed to a wedding scene, or to a couple of characters acting as if married. Actually I'd like that, but I'm not sure it could be justified to have those kinds of scenes in-game, when there are so many other areas that need the resources, animation, writing, etc. devoted to them. Guess what I'm saying is if the rest of the plot, dialogue, etc. suffer, then it's a huge no from me--but otherwise, of course, I like the idea.