Consider this line of thought.
- To decide to marry y for no reason is unreasonable.
- To decide to marry y on the grounds that y will make one happy is selfish.
- To decide to marry y on the grounds that one will make y happy is arrogant.
- To decide to marry y on the grounds that it will make the world a better place is to be full of oneself.
- To decide to marry y on the grounds that God is calling one to it is unavailable to atheists and claims an implausibly good understanding of God's designs.
I am inclined to think that (5) can be rejected: I think atheists can do something because God calls them to it, but they won't formulate the reason in that way. I also think we can have some understanding of what God calls us to. And while (3) and (4) may be true, perhaps all one needs as grounds is the thought that likely one will in a small way contribute to y's happiness and make the world a better place, and one need not be arrogant to think that.
Still, I think that even with these qualifications, something important is being left out of the story. Love, I guess. Love directly justifies the pursuit of a union appropriate to the love, and marriage is the union appropriate to certain forms of love.