You claim that being naked in public is no big deal. And as I recall your response to children was that seeing someone naked would not traumatize them enough to need years of therapy.
THIS society has mores and accepted behaviors. Naked in public is NOT one of them. YOU have no right under any Constitution or law to go naked anywhere you damn well please, nor have sex in public.
You're evidently unfamiliar with the logical parameters of ethical discussion and debate, so let me begin by telling you the difference between "is" and "ought," or more clearly, the difference between a "descriptive" observation and a "prescriptive" recommendation. "This society" may indeed have "mores and accepted behaviors," but their mere existence does not function as an ethical justification of them any more than the prevalence of segregation and Jim Crow laws provided an ethical justification for them. Mere existence is far from being a sufficient justification.
That would be WHY we have those laws and we do not repeal them. Sex in public is already a problem in some places because of male gays. They leave their unsanitary left overs in those places. This can cause disease and sickness. And since it tends to be PARKS where children are supposed to be able to play it endangers them since most young children have no idea what that crap is.
Your personal desires do not trump the Majorities distaste and the unsanitary problems of what you think is just fine.
Then sanitation can function as your concern, though you still lack an ethical objection to "sanitary" forms of public sexual activity. For instance, your objection would clearly not retain even a slight semblance of logical soundness in the case of persons who "cleaned up" after themselves or endeavored to prevent any contamination from occurring in the first place. You're going to have to provide a more plausible objection, and one significantly more substantive than cries of "protecting the children." The conception of children being harmed through viewing sexual acts is a fairly recent societal phenomenon that has not existed for more than a few centuries. Of course, keeping in mind the difference between "is" and "ought," that alone does not function as an ethical argument, but it behooves you to provide one nonetheless.
Do you have a logically sound objection to express, based on empirical evidence that visual exposure to sexual acts psychologically harms children or anyone else?