In brief response to Eugene's thoughtful post below:

I. The Yuck Factor in Public Space.

Eugene argues that perceived yuckiness should be sufficient to justify regulation of behavior in public space because the space is government-owned and the state has a right to "maximize the aggregate enjoyment of those spaces." I am not convinced that this is sufficient justification. If it is right for the government to ban any behavior in public spaces that the majority considers "yucky" (in the absence of explicit constitutional protection), then - at least in some jurisdictions - that would justify bans on a wide range of activities, including, for example, public handholding between same-sex couples. Moreover, at least as a moral matter, I don't see how Eugene's argument would justify forbidding the state to ban offensive public speech or restricting the presence in public spaces of people belonging to unpopular racial or religious groups.

True, such laws are forbidden by the Constitution, but Eugene's analysis seems to imply that the Constitution is wrong to forbid them. After all, if the speech or the groups are hated by enough people, banning it (or them) might "maximize the aggregate enjoyment of [public] spaces." Furthermore, I'm not convinced that a categorical ban on public nudity and sex really does maximize aggregate enjoyment. Those who engage in such behavior (even when it is legal) are braving severe public opprobrium and social pressure. They are willing to pay a high cost to do what they do. That suggests that they derive a high degree of utility from their activities - possibly enough to outweigh the disutility to others caused by yuckiness.

Finally, real world governments are unlikely to limit themselves to banning only those yucky behaviors whose absence will maximize aggregate enjoyment. They are instead likely to respond to pressure from interest groups and ban many activities whose yucky aspects do not in fact outweigh their benefits. This is especially likely to be true if we keep in mind the fact that there is no good way to measure how "yucky" a given activity really is and how much disutility a given amount of yuckiness causes.

II. Unwanted Sexual Arousal.

Eugene also argues that public sex and nudity can be banned because they lead to unwanted sexual arousal in bystanders. It might be true that this sometimes happens, but I highly doubt that it is the real reason for most opposition to public sex and nudity. My guess is that the "yuck factor" explains about 90% of the opposition and fear of excessive arousal perhaps a fraction of the remaining 10%. Moreover, there are many other activities that arouse strong, but unwanted emotions in bystanders. Flag burning, for example, arouses very strong feelings of anger and hatred. In a diverse society, almost any activity will generate strong unwanted emotions among at least some people. As a general rule, the management of emotional reactions is unlikely to be a task that government will do a good job at. It is a classic example of a matter best left to the private sector.

OK, I think that's enough (some would say more than enough!) yakking about yuck.