It is when evil is consented that it does spiritual damage.
If evil entities torment people without their consent, the person may be spiritually enhanced by the experience. This is why wars may (like WWII in the UK) apparently lead to a Christian revival - as people fight evil.
But evil consented-to is evil internalised - like with the sexual revolution.
The magnitude of an evil is, in the end, less important than whether it is resisted or embraced.
An embraced trivial sin is generally far more harmful to the soul than a major sin repented. Even a trivial sin unrecognised as such, leads inexorably to more and worse sin; but a major sin repented is not harmful to the soul, and may be beneficial (St Paul, for example).
Sex is probably the second-most-powerful motivation in 'natural' men - religion being the first. Now moderns have dispensed-with (real) religion; for modern Men, sex reigns supreme as human motivator.
Most of the sexual sins are relatively trivial, in the great scheme of things. Therefore, modern people say: why not? Unasked, since they deny God in their hearts, is the proper question of whether some-sexual-thing is what God could plausibly want from them?
The danger is that with the sexual revolution what starts as why-not-it's-trivial? remains unrepented; then becomes no sin at all; then - since there are only two sides in the spiritual war - then becomes a virtue (to be defended, celebrated, funded, enforced...).
In such a situation, even a trivial sin rapidly becomes spiritually lethal; as we see all around us.
The Big Problem with the sexual revolution is, therefore, the combination of relatively-trivial sins combined with powerful motivation to embrace one or more such sins.
Obsessive desire synergizes with a perfect excuse to yield pride-full self-damnation and public advocacy of the same for other - which is an extremely evil result.