My interpretation of this is from the perspective that repentance is something close-to being the core value of Christianity. The Liberals are correct that God does not (it would be ridiculous) expect perfect behave from us, and that lapses (including sexual lapses) are forgiven.
But this absolutely requires repentance.
So when we behave suboptimally we must repent that failure and must not defend the lapses - must not say that our failures are not-really failures.
The trend for the past two or three generations has been to assert that sexual morality is not such a big things as all that - especially, it is not the core value of Christianity, and is sometimes over-emphasised. Other things are more important in and of themselves. Sexual sinners are not the worst kind of sinners. Sexual virtue is difficult, and nobody wholly achieves it (in the privacy of their own minds). Hypocrisy is rampant.
All this is true - but that merely means that failure in sexual morality is is frequent - it does not mean that failure in sexual morality is unimportant; and it certainly does not mean that failure is actually success!
If (or when) we do make this claim that sin is not sin - then that itself becomes a further sin which itself requires repentance.
When I say any sin (including sexual sin) 'must' be repented I mean this as a personal principle. It is not a matter of God punishing us for failing to repent - it is that by failing to repent we choose to reject God; we choose to reject God's created order; the actual failure to repent is itself the rejection.
We thereby damn ourselves.
Failure to repent - even minor or sub-primary sins - simply is, in itself, our personal choice to reject salvation.
Since choice (free will/ agency) is a real fact of Life, then this represents self-damnation.
In fact this is what damnation actually-is.
The gates of heaven are open - that was the gift of Christ; but we have-to want-to go in under our own steam, we cannot be shoved-in.
Failing to repent - that is, failing to acknowledge-the-reality-of our sins and other failures - is merely a roundabout way of rejecting divine order, of saying that we personally do not want what God has to offer - and preferring to 'go it alone'.
And if that is what we want, that is what we will get.