The thing is, what determines when autonomy is absolute and inviolable, and when it should be weighed against other concerns?

When it comes to interventions in human affairs by the state and other institutions, modern liberals pride themselves on their supposed adherence to (what they see as) rational and scientific cost-benefit analysis and common-sense notions of equality and fairness. They typically assert that their opponents are being irrational, or acting out of selfish interest, when they insist that some other principle takes precedence, like for example when conservatives insist on respecting tradition and custom, or when libertarians insist on inviolable property rights. In particular, liberals certainly see it as irrational when libertarians oppose their favored measures on the grounds of individual liberty and autonomy.

However, there are issues on which liberals themselves draw absolutist lines and lose all interest for cost-benefit analysis, as well as for concerns about equality and fairness that are perfectly analogous to those they care about greatly in other cases. Sex is the principal example. Liberals argue in favor of comprehensive intervention and regulation in nearly all areas of human life, but in contrast, people's sexual behavior is supposed to be a subject of complete laissez-faire. This despite the fact that many arguments that liberals normally use against the evils of laissez-faire and in favor of economic intervention, wealth redistribution, and paternalistic regulation, would apply with equal (or even greater) force to sex as well. Yet an attempt to argue in favor of more restrictive sexual norms on any of these grounds will be met with immediate hostility by liberals -- often so fierce that you'll be immediately dismissed as obviously crazy or malicious.

I don't think it's possible for liberals to salvage the situation by claiming that sexual laissez-faire is somehow entailed by the same considerations that, according to them, mandate complex and comprehensive regulation of almost everything else. This would be vanishingly improbable even a priori, and a casual look at the arguments in question definitely shows a glaring inconsistency here. The only plausible explanation I see here is that, just like everyone else in the human history, liberals base their sexual norms on a sacredness foundation -- except that for them, this foundation has the peculiar form of sacralizing individual autonomy, thus making a violation of this autonomy a sacrilege that no other considerations can justify.

Ironically, the sexual norms based on sacralized individual autonomy end up working very badly in practice, so that we end up with the present rather bizarre situation where we see an unprecedented amount of hand-wringing about all sorts of sex-related problems, and at the same time proud insistence that we have reached unprecedented heights of freedom, enlightenment, and moral superiority in sex-related matters. (And also a complete impossibility of discussing these topics in an open and honest manner, as witnessed by the fact that they reliably destroy the discourse even in a forum like LW.)