Jud, the prophecies regarding TPTWP and TDHTH (The Dragon Has Three Heads) never mention sex. And I think I'm the person Arsten is recalling from his last post. If the Targs are to reestablish their dynasty, then they must procreate. Since Dany assumes she is barren, then she must pick one male rider and one female rider. The prophecy is not called There Are Three Dragons... the prophecy is THE Dragon (this means one dragon) Has Three Heads. I feel this is strong evidence that the three humans are not merely riders, they are the emblems of unified rule, i.e. Royal Authority. To me, this means they are married... as Visenya, Aegon I, and Rhaenys. I do not envision Dany allowing two riders who are not spouses... that would be There Are Three Dragons. If she finds a male Targ rider and she herself is barren, then the other rider/spouse must be the surrogate mother. Riding dragons, like demigods, is a sure sign to the masses that not only are the Targs physically above humanity, but they are also above human morals, i.e. polygamy becomes justified. But we've learned through TWOIAF that non-Targs also rode dragons.... so anyone capable of riding a dragon could easily be seen as a candidate for the throne. I just don't think Ben aspires that high, nor do I believe Daario can convince either Rhaegal or Viserion to allow him up.