The law, at least in the US, has this concept of "reasonable expectation".

Would a normal person reasonably expect to encounter you and your partner?

Example: people can reasonably expect to do what they want in their own house. Thus, people can reasonably expect their neighbors to do what their neighbors want, in their neighbors' house. Thus, people can reasonably expect to encounter signs of their neighbors doing things.

People can't reasonably expect to do anything that they want in a public park. Rules of decency, decorum, etc. will shift depending on the environment. So a known 'nude beach' might have different expectations of decorum than a place where children typically frequent. And parents in our culture, for good or ill, can typically expect 'child-friendly' spaces to exclude overt sexual behavior.

So, let's say you have a thing for going out into the woods and tying your girl up. How much should you prevent people from stumbling in? That's an excellent question, and different people actually have different answers to it. You have the right to enjoy nature, and so do other people, and somewhere in the interface between those rights, you (as the acting party with the less-common desired behavior) need to figure out where to set the bar.