You’re approaching the question from a post-Enlightenment perspective, which is something that doesn’t exist in Azeroth. Azeroth is pretty squarely pre-Enlightenment. So in-game, it’s entirely normal and natural for everyone to expect a King to need a Queen to get the job done.
There are other ways to get around the issue, and adoption is absolutely one of those ways – look no further than Rome’s Five Good Emperors to see a fine example. Arguably the best string of Emperors Rome ever had came about as a series of childless Emperors adopting an able lieutenant as heir. Worked great until Marcus Aurelius had to go and have Commodus as a son. (Not saying the Crisis of the Third Century was entirely Commodus’ fault … but it was pretty much Commodus’ fault.)
But we’re not Romans. So we need a Queen.