There are various lines of argument that point to the since fact: modern Man has corrupted instincts.
By this I mean that when modern Man does 'what comes naturally' he does not behave in an adaptive way. Adaptive means fitness enhancing; means increasing of reproductive success.
So modern Man, behaving 'naturally' is Not a 'healthy animal' but a sick one, a demented being, a crazy creature.
This is something new. There used-to-be a thing called the Natural Man, who had a natural morality - and upon-whom Christian morality could build; but this is no longer the case.
There are several materialist/ scientific reasons why this might be so; such as mutation accumulation, the multiple and combined toxicities (chemical, hormonal, electromagnetic...) of the industrial society, the evolutionary 'mismatch' between the society we evolved-in and the society we live-in, and the saturation propaganda for evil and unnatural behaviour from The System of bureaucracy and the Mass Media.
There are also spiritual reasons: the idea that we are meant to, divinely destined to, be attaining a higher form of spiritual consciousness; but are (en masse) refusing to do this; and are therefore unwitting victims of unconscious and distorted instincts.
The corruption of instinct is the end of traditional forms of religion - of those many forms of religion based upon an authoritative church structure and the obedience of the adherents: the large-scale traditional forms of (for example) Eastern and Western Catholicism, Anglicanism, Calvinism and the Presbyterian, Methodist, Baptist and other churches.
To varying but vital degrees - all of these depend upon a baseline of unconscious consensus about 'natural' Good and evil, beauty and ugliness, truth and dishonesty. This consensus has been destroyed along with the instincts that supported it.
And this corruption of instinct has happened soonest, most powerfully and more thoroughly among the church leaders; thereby accelerating and increasing the problem among the masses, the laity, the followers.
Thus, everyone is corrupt; but the corruption is made worse by the fact of its being worst among those with power, status and wealth.
Nowadays, everything must be made explicit and fully conscious; including the assumptions which used to be taken-for-granted.
This has become most obvious in the area of sex and sexuality, where what used to be regarded as good, desirable, virtuous on the basis of common sense assumptions; are now subverted and inverted because the common sense is either become feeble or itself inverted.
We need to go as deep as our primary (metaphysical) assumptions, to know them; discern and decide - decide not not by common sense, nor by instinct - which is gut feeling; but by intuition which is the discernment of fully conscious, primary thinking of the real and divine self.
Indeed, before we can even attempt this, we each need to have decided that it is coherent and possible; that there is a part of ourself which is divine and which can know - know directly and without mediation - the truth of things.
We must - that is - be able to distinguish between instinct - which is thing of the animal in us; and intuition, which is a thing of the divine in us.
Instinct cannot save us - but will, on the contrary, direct and drive us into damnation and death; but divine intuition can save us; and it is the only thing that can save us.