That was an odd episode. Looked like Catherine was a goner just from the editing (clouds and thunderstorms eclipsing a bright sky). But he's preached the whole family thing from day one, and she doesn't have a family that would welcome him. So they must have much more than we're shown or she would have gone long ago.
He's not a math nerd, by the way. 13x13? No. He'll just have to accept, if he chose her, that she's the one with the math skills.
Lindsay is the safe choice, it seems. The one with the perfect family for him, the one who would work hard to be perfect for him. Is that enough? She seems to be willing him to choose her.
It's an odd season. Really good casting, more realistic story lines than usual. I suppose we'll all be disappointed for the 17th time (does Molly count, being second choice?) at some point down the road. But they've done a much better job than usual telling the story.