marriage extending into the afterlife.

And I think this very question is what led them to polygamy. I think if you pushed them on it, they'd still say that if a man had multiple marriages (chronologically separated, of course!), they'd say they all join in a polygamous marriage in the afterlife. The question of what happens if a woman's husband dies and she remarries, I imagine, is one they'd try to avoid.

(Of course any reasonable cynic would view the causation the other way around. Joseph Smith wanted more wives, and this was the argument he came up with for it.)