DarkForces
I can't believe it took me so long to figure this out, but Ye Chen's grandfather was likely a cultivator. Like he knew Ye Chen had an Ancient Yang body and would normally die early due to that, but when he saw that Xia Qingyu had a pure yin body he made such a strange request to save Ye Chen's life. He probably thought Ye Chen would never step foot into the cultivation world, so he set Ye Chen up with a woman who has extremely strong Yin to balance out his yang. If we want to go even further down this rabbit hole it is even possible that he knew whoever Ye Chen married would die from having too much of Ye Chen's yang, which was one of the reasons for a harem. So he chose someone with strong enough Yin energy so he wouldn't become a widower or even worse a murder suspect if all women he had long term relationships with died due to his extreme yang energy.