The title says it.
The core reason why Leftism, Liberalism, Political Correctness and the Social Justice Warriors have been winning and winning and winning for fifty years and more - is their offer of Sexual Liberation.
The Left offers Sex - whatever you want, potentially unbounded, and religion does not: all religions are restrctive about sex - and the mainstream socio-political opposition to the Left (the secular Right) are perfectly clear that they want the sex more than they want anything else.
It really does not matter how psychotic life becomes in the West - how obvious are the absurdities of Political Correctness - how wicked are the inversions of truth, beauty and virtue... there will not be and cannot be a backlash against Leftism, except on the other side of a mass religious revival.
It is precisely because the Left absolutely and categorically refuses to be religious that we have Leftism. No matter what happens, the West stubbornly and categorically refuses to be religious.
(I mean seriously religious - religious so that it makes a difference, so that it leads to significant self-sacrifice; not being 'spiritual' as part of a lifestyle, nor being 'liberally' or 'moderately' religious = primarily Leftist with religion fitted-around.)
The West refuses to be religious because of Sex - because all religions demand sacrifices when it comes to sex; and Westerners will not make these sacrifices.
Nearly everybody in The West has their own sexual 'thing' which they most hope for, and which religion would take away from them (or make them feel bad about) - sex before marriage, sex with lots of people, extramarital sex, sex with anybody or anything other than your spouse...
Most people don't ever get what they want, sexually, but they live in hope of it - and religion takes away that hope; takes away their 'right' to live-by that hope.
And that is intolerable to Modern Man. Modern Man will tolerate anything rather than that - including the collapse of the system which provides him with peace, prosperity, comfort, convenience and distraction.
Ultimately Modern Man will not get what he wants - because that is impossible in this mortal life, and that kind of wanting puts each individual at odds with every other human being, and the whole of creation.
I'm not saying that the one and only reason that each and every secular Western Person who rejects religion has done so because he will not give-up his or her 'rights' to some religiously-forbidden sexual possibilities.
Not quite everyone, there are probably a few exceptions, those whose rejection of religion is principled and un-self-interested and really-and-truly nothing to do with sex, not fundamentally based-on sex, not even deep down in their secret fantasy life...
Maybe you are one of them; but probably not.