For that matter, why do Varys, Tyrion & Davos think Jon & Dany will be resistant to the idea of marrying one another? From the way they were talking about the couple, you'd have thought they intended to propose Jon & Dany marry other people, and their affections would be an obstacle. And since when is Davos the sort to scheme and plan marriages, much less talk politics with people on Tyrion's & Varys' level?

People balk at glaringly obvious ideas to generate conflict and delay the resolution to the point in the season where the writers want it to happen, and they talk with people they wouldn't, about subjects they wouldn't, because there has to be exposition.