You know what topic came up a lot during Episode 6 of Game of Thrones? Babies. Yes, babies. Human babies. Monster babies. Dragon babies. Then there's the fact that Daenerys seems to think that she can't have any human children of her own — but is that the case? Could Daenerys get pregnant on Game of Thrones? Book-theories and show-theories differ somewhat, but many people believe that Daenerys is going to have a bun in the oven very soon.
Daenerys' fertility problems go all the way back to Season 1, after Drogo died and then Daenerys unwisely convinced a witch to bring him back to life. The witch convinced Dany to use blood magic, and Dany ended up exchanging the life of her unborn child for Drogo's (his resurrection required a sacrifice). But Drogo didn't come back to life as himself. Instead he came back as a lifeless vegetable. Daenerys asked when he would be like himself again and the witch, named Mirri Maz Duur, said, "When the sun rises in the west and sets in the east. When the seas go dry and mountains blow in the wind like leaves. When your womb quickens again, and you bear a living child. Then he will return, and not before." Since Drogo is definitely dead, Daenerys logically took this to mean that she will never have another child.
But some people don't put that much stock into this prophecy. After all, the show did omit that last part about the womb, though Daenerys believes herself to be barren. The sixth episode in particular seemed to focus a lot on the state of Dany's alleged inability to have children. She and Tyrion discussed succession, and who will take her place when she's gone. Then in keeping with the child theme, Jorah kindly told Jon that the sword Longclaw (the ancestral Mormont sword Jorah's father gave Jon) will be passed on to Jon's children. And then Daenerys informed Jon that the dragons are the only children she'll ever have. "Do you understand?" she asked him. Honing in so much on Daenerys' supposed infertility, as far as storytelling goes, seems to imply that she's potentially not infertile at all.
If she and Jon do hook up, as they are bound to do, what then happens if Daenerys becomes pregnant? They could solve some sticky political problems by getting married. But what happens when they find out that they're related? How would this impact their relationship? As cool as it might be, this possible plot twist could come with some consequences if it ever comes to fruition. Which it may not. But come on — this is GoT we're talking about so anything is possible.