Thanks to DarkUFO reader Jason for the following.
I was re-watching LAX and thought that if Desmond was on the plane (and was actually there and not a figment of Jack's imagination), then that meant he never went to the island (especially if its at the bottom of the ocean haha). And Desmond not on the island has a much greater chance of marrying Penny than Desmond on the island.
And thus I found this...
It appears our man Des is a (happily?) married man in this alternate timeline. Very exciting.