It seems that sexual depravity is the most insidious vice. Everyone knows that substance abuse, violence, and dishonesty are harmful, as their negative effects can be immediately seen.
But fornication, adultery, lust, are easier to rationalize. With popular culture worshiping capital-S Science, people (especially the young and impressionable, who are already going to be sexually and likely spiritually frustrated) are bombarded with claims that "it's just natural bro, it doesn't hurt anyone". And since there's no immediate evidence of a negative effect, it's easy to believe that.
Even among Christians, it's easy to use some idiotic reasoning to pretend it's not sinful. I used to watch porn and jack off regularly, and I'd tell myself that it's not as bad as fornicating, or that nobody involved was getting hurt since they consented to it, etc.
The effects are much more slow-burning than other vices. As Augustine said, indulgence becomes habit, and habit becomes necessity.
Addiction to sexual degeneracy dehumanizes others, reverts oneself to base physical hunger, and cripples the development of any love and enlightenment that have been developing via virtuous living.
A sinister and creeping problem needs to be wrenched from society more urgently than one still considered by "common sense" to be bad.