Morals are (at least in part) based on generalising instinct. Most people would be instinctively repelled and outraged if they saw an adult torturing a child – morality is not that instinctive feeling itself, but when we reflect upon that feeling intellectually, especially when in doing so we universalise it, moving from "I am repelled and outraged by this, so I won't do it and do I what I can to stop others from doing it" to "everyone ought to be repelled and outraged by this, therefore nobody ought to do it and everybody ought to do what they can to stop others from doing it", that's when we've turned instinct into morality.

Is morality "entirely subjective"? When someone says, "Torturing children is immoral", do you really want to reply "That's entirely subjective". The very reply itself seems morally compromised, even immoral.

A good philosophical argument for the objectivity of morality is the "companions in the guilt" argument. Most people accept the objectivity of rationality, even those who deny the objectivity of morality. And yet, when we examine the two in detail, we can find a great many similarities between rationality and morality – morality says "don't do X", where X might be torturing a child, rationality says "don't believe X", where X might be the QAnon conspiracy theory; morality values states of affairs positively or negatively (moral vs immoral), rationality does the same (rational vs irrational) – in light of these similarities, how is it not special pleading to affirm the objectivity of one yet deny it of the other? To justify treating them differently, you'd need to give a good reason for doing so – but rarely is any reason given at all, and I don't think any of them are good. Rationally, we must either affirm the objectivity of morality or deny the objectivity of rationality. I think, to affirm the objectivity of morality is the better horn to take – we might even conclude it is the more rational one, if we believe that rationality itself requires belief in its own rationality. Hence, morality is objective.