Preventing same-sex couples from adopting a child is an act of discrimination based upon religious belief

There is no evidence to support that a child is at risk of harm because the parents adopting them are same-sex. Imposing religious beliefs upon any persons is a violation of human rights and is unethical. Simple as that.
But if you want to talk about morality, prevention of care to a child based upon a predicated "sacred law" is immoral and only furthers the "us vs them" hatred cycle.
And if you want to talk about authority, Jesus frequently and publicly rebuked the religious leaders of his time for doing exactly that. Imposing rules and feeling superior because they thought they were being more "righteous" by following them.