Xia Feng must learn more about yanderes - there might be more in the erossu book after all.
It was not a love, but just a pure emotion that wished for her well-being.
Xia Feng had no way of knowing Valaraska and that she is a celestial idol, but he was just feeling a little sad to see such lady being betrayed. It was not her fault for wishing for the one man for a whole life while expecting the same from her chosen one.
But even if there was a rule of Reincarnation Emperor to not go for many women, everyone was still breaking it as it simply can raise their powers. Many women allow for many connections which are at the same time way more stronger than an alliance between 'two men' based on the words alone.
Such connection can give a birth to a descendant and this descendant also can just straighten the ties between two forces...
There was also a bloodline concept where many people can raise their abilities just by indulging themselves in the pleasure of their yin and yang qi.