Good point. But I don't see Jon marrying Dany. I don't see her marrying again. And she's barren at this point anyway no? No heirs to be had.
I feel like a broken record on this point at this point, but no, she is not barren. She was told she would not conceive again until such and such seemingly impossible time. If you interpret the impossibities literally, she is barren, but fantasy witch's prophecies are never literal. She'll conceive when the sun rises in the west and sets in the east means she'll boink some dude with a rising sun on his family crest who arrives from the west and sneaks out the eastern door in the morning.