Likely the angels and demons themselves are the ones who disapprove, rather than the Gods. Masses often disapprove of breaking tradition, even if a powerful leader has done so before.
But Merlin isn't their child tho. She's called their blessed child because she received gifts from both of them. Ch 249 states that when Merlin was born both the goddesses and demons quickly heard of her cas she was born with the power of Infinity which made the supreme deity and the demon king both seek her out cas they wanted her to join their faction. In ch 243 Merlin also talks about her dad who was a sage.
Nah ... She was just born with infinit magic power .....
And both the demon n god side wanted that powerful kid she was
those blessings where kinda like a bribe to her ... So that she joins their side ...
But well.. As we all know .. She betrayed both of them
If u read the chapter again .. U will understand (≧∀≦)
Ya know~ I might be dumb for barely asking this now, but if Merlin is the love child of Goddess and Demon God, why can't Meliodas and Elizabeth be together? Well I know there is the logical possibility that the two (goddess and God) had a relationship and thought "oh we're too different for each other so we can't be together" but I can't help but wonder about it. （￣へ￣）