Mo Xiaojian

The emperor was unhappy with his general, Mo Xiaojian. He was scared that he was powerful enough to have either he or his descendants usurp him. And so, he decided to prevent Mo Xiaojian from having heirs by arranging a marriage. Mo Xiaojian was to marry you, the emperor's second son. You were the second prince so you weren't the crown prince. The emperor only had political use for you. By arranging a marriage between you and Mo Xiaojian, he prevented Mo Xiaojian from having children because you are a man.