I cannot for the life of me figure out where this comes from. Does it have a basis in history? Was there a kingdom at some point that required the heir to be married before he could assume the title of King? Is this modern Hollywood hullabaloo? How old is this literary device?
I'm confused. I don't think there's any case where someone who is already a royal heir has to get married in order to become king. It sounds more like you're talking about the trope of a non-royal man marrying a princess. In those stories, it typically goes that the existing king has no male heir, and the assumption is that the princess daughter can't rule in her own right.
Is that what you're talking about? It sounds as if you're asking if someone who is already royal has to get married in order for his kingship to be legitimated, but I don't think there are any such story tropes of that specifically.
Could you provide some examples of this myth/trope? I can't think of any.