"There was a legend in our clan..." Aunt Yue started the story in a soft compelling voice.
In that legend, it was specifically said that the pure Bei Yue could activate her power once she mated to her one true love. How could they know whether it was really her one true love or not? It was from the glowing birthmark, of course. The Bei Yue clan had an affiliation with the moon, especially the crescent moon. That was always the symbols of their clan from generation to generation.
If Yao Ling made love with another person and he wasn't her one true love, then the birthmark wouldn't glow. They wouldn't be able to activate her power and their bond and it seemed like this was used to prevent that people would use Yao Ling's body just to achieve his ambition.