Actually, since this is a pseudonym, I've had sex in public a several times and never had a problem. In fact, it was in one of (Saratoga, Los Gatos, Los Altos) wealthy communities of mostly residences that have little real crime, and locations were even suggested by the police (no they weren't filming). We even asked what they would do, they indicated there would generally not be a ticket unless it were somehow indecent (visible to others) and they just make sure it is consensual. It might've been the luxury sportscar at the time or the hot girl might've had something to do with police being extra helpful. (Btw the hot girl broached the questions, hilariously.)
It would be different, for say two shabby-looking folks going at it in Venice, CA in front of a street during the daytime. But a hot couple, at night, in some remote location in a safe area isn't going to raise eyebrows, especially if no one notices. It just goes to show there are layers of socioeconomic biases, plus not all homeless people are given feedback, care or are aware of what's considered acceptable by others.
Perhaps the core issue is should necessary biological functions (and common, strongly desired needs that others are able to fulfill) be disproportionally criminalized based on socioeconomic and other biases? (For most men, sex is a strong motivating drive... which is why the Netherlands includes it in public benefits.). There might be a "time and a place," but people whom have nothing need equal access to water, food, sanitation and whatever else can be afforded to make their lives a little more comfortable. (A society is judged on how it treats...)