Why the automatic hostility towards the idea that under sexual laissez-faire, a huge segment of the population, which lacks sufficient prudence and self-control, will make disastrous and self-destructive choices, so that restrictive traditional sexual norms may amount to a net harm reduction? Especially since liberals make analogous arguments in favor of paternalistic regulation of practically everything else.

I don't know about automatic (and I am not presenting my own position) but it is certainly legitimate for a person to be hostile to being coerced into a worse situation because someone else believes (even correctly) that other people will benefit from said coercion. Similarly, it is hardly unreasonable for the one person who is being tortured for fifty years to be hostile to his own torture, even if that torture is a net benefit to the population.

If you want to do harm to people (whether paternalistic control or counterfactual torture) you should expect them to fight back if they can. Martyrdom is occasionally noble but it is never obligatory.