Shen Nianzu could literally envision what would happen if Jin Jiuchi's identity as a nightmare creature was exposed—
They would be horrified, yes. They would be horrified and appalled through and through. Not only with the prospect of a nightmare creature mingling among humans, but also with his and Jin Jiuchi's romantic relationship. They might even assume that Shen Nianzu had been coerced or bewitched into it.
No matter what it was, they would only expect the worst in Jin Jiuchi without taking into consideration his character, temperament, or his behavior so far.
All because he was a nightmare creature.
Because he was fundamentally different from them all.
Shen Nianzu saw no way they could have accepted Jin Jiuchi into society. They would forever shun him, be wary of him, always watching for the day he would reveal his 'true color' and turn against them.