More times.
Sylvanas had probably too close relationship with Nathanos Blightcaller, what would make all sisters Windrunner weird and perverted about human males. This is just not natural. Gorillas aren't mating with orangutans, and dogs aren't mating with cats. Even if sapient race would fall in love with other sapient race, there should be no children from this relationship - just like there should be no Arathor the Redeemer and no Rhonin's and Vereesa's kids. And no Med'an, too.
And Kael'thas was in love with Jaina, too, what is weird, because as a heir of Quel'Thalas throne, he should mate a woman from his own race, the noble woman, of course. But I hope he only wanted Jaina to bed, not for wife. Anyway that things just shouldn't happen, because what will be next? Human-draenei? Orc-tauren? Troll-gnome? It would the best way to produce more Med'ans. And this is why romance between Khadgar and Cordana would be very bad, but if she is just his bodyguard - it's all ok.