The simple answer is not primarily the things themselves, not the effect of Leftism* (including the sexual revolution - which is Leftism's most insidious and effective manifestation); but the failure to repent.
That is, the failure to know that these are wrong. The failure to recognise, acknowledge that these are not The Path; that these are not what we should be doing - that these are temptations towards sin.
After all, we have it from Jesus that sin is no barrier to salvation, nor even to theosis (to becoming more divine). But for sin to be negated, we must know that it is sin; and to know sin, we must know goodness, virtue, beauty and truth.
The special, lethal damage of Leftism is that so many people are so deeply persuaded that some particular (or more than one) of the multifold Leftist ideals, principles, rights, prohibitions, aspirations... are needful.
(By which I mean any or all of the mutating and evolving imperatives of Leftism: pacifism, abolition of slavery, equality, socialism, social justice, feminism, antiracism, abolition of suffering, the rights of non-biological or pathological sexual orientations and aspirations, environmentalism, libertarianism, transhumanism... etc. and so forth.)
Needful and necessary in an ultimate fashion - as a bottom line.
Necessary for themselves, necessary for some others or groups of others - needful in a kind of cosmic way.
Either that - or another kind of Leftism; that these matters (political, social and sexual) are - as a matter of principle - trivial, unimportant, nobody's business, arbitrary... that they are purely materialist arrangements of matter; excluded from the realms of values; nothing whatsoever to do with truth, beauty or virtue.
What absolutely blocks real Christianity among Leftists is the failure to include the political, social and sexual within the reality of Life that is relevant to following Jesus. This is not, at root, a matter of following rules; but a refusal to be real.
To know and follow Jesus, to know one's mortal life in the context of Heaven to come; is to regard this our mortality as really-real in a way that is absolutely clarifying of our needing to have faith and love in the first place; and then to live this life and learn from it.
What we have, endemic in The West, is a mass refusal to learn from life; derived from a failure to regard life as real, significant, solid and of Heavenly scope. Leftism (including the sexual revolution) is a commitment-against this.
So, in a world in which Leftism encompasses almost the entirely of public, social, institutional life; in which the sexual revolution has been inculcated, encouraged, rewarded since infancy - we are all of us prone to temptation and lapsing. We are all complicit. Purity and consistency is unattainable. We are all infected and diseased, to some significant extent.
None of this matters when we are prepared to live and learn, to recognise, know and repent (trial, error, repentance; and trial...); and this is God-given and comes spontaneously and naturally (if we but allow it - albeit few do) - for those whose orientation is set-upon following Jesus through death to Heaven.
We will stray from the path, we will know when we stray - we can return to the path and continue; but only if we personally and consciously choose to make Jesus primary, and acknowledge when we have strayed.
*Note: This question about the lethal effect of Leftism on real Christianity is of central importance to our time and place: The West, here-and-now; because Leftism is the entirety of mainstream public discourse - including all conservative, republican, libertarian, nationalist groupings... Any and all discourse that does not acknowledge the primacy of the divine is Leftist - to put it another way materialism is Leftism. Therefore, all who call-themselves 'Right' yet who are not primarily religious in terms of how they aspire to arrange society, are actually of-the-Left. The Left (of all types) is therefore the party of evil, whatever one's religion. There are - of course - many religions with very different ideas concerning the nature of deity - but the question of by which religion we ought to live, is a separate discussion.