At the end of the day, the preservation of Yuan Yang is indeed a factor beneficial to cultivation.
But what Wei Dongliu was reluctant to admit is that he did possess a certain moral fastidiousness—albeit only in the aspect of treating a companion.
He didn't want Jiang the Witch to think that the man to whom she had given her body was Wei Dongliu.
Though it may seem strange: if becoming Daoist Companions was already Wei Dongliu's role, then why couldn't the one to whom she gave her body be Wei Dongliu?
Of course, it's because the former was just a cultural ceremony, whereas the latter was a permanent imprint upon the body… In any case, he was quite steadfast in this principle and would not waver.
Jiang the Witch naturally couldn't understand this, so she indeed tried every trick in the book.