When it comes to ethics there aren’t any right or wrong answers; what is considered ethical by some will be considered unethical by others, and determining the ethics of an activity will always depend on multiple factors. Ethics is a way of digging into a topic and exploring it deeper, it isn’t a path to universal answers or solutions. With that caveat in mind, here’s my answer.
I think that the relevant ethical question regarding sex in public is one of consent. Consent is a useful place to start because I believe most people would agree that non-consensual sex (which should not be confused with a fantasy of non-consent) could be considered one form of unethical sex. If you agree that forcing someone to have sex isn’t ethical, it follows that ethical sex must include consent of everyone involved.
With that in mind, consider this situation. I choose to have sex in a public place, let’s say a baseball field in a public park on a beautiful Saturday afternoon. A stranger happens along and sees me having sex on first base. At the very moment they are looking at me, I look up and make eye contact with them. In my heightened state of arousal I feel somehow connected to this person, as if some very private part of me has been exposed to them. Is this person implicated in my sex act? They aren’t literally having sex with me, but by witnessing and responding to what they see (whether that response is one of fear, anger, arousal, intrigue, or giddy laughter) I argue that they are engaged in the sexual act taking place, whether they like it or not.
Which is to say they are engaged without their consent. They didn’t know, nor could they really have imagined, that walking through their favorite park on a Saturday afternoon, that they’d be watching me have sex. This seems to be to be non-consensual, and therefore unethical.
I want to qualify this ethical judgment I’ve just made. First, I want to make it clear that just because one person finds something unethical, it doesn’t follow that it’s “wrong” or that people shouldn’t do it. So in saying that I believe sex in public is unethical I’m not actually saying I don’t believe people should ever do it (although I would strongly advise against sex on first base in a baseball diamond at a public park on a Saturday afternoon).
Second, I want to acknowledge that context counts for a lot, and if you change any one of the variables in the above story I might arrive at a different conclusion.
Finally I think that the concept of reasonable effort should apply here. I think that if you want to have sex in a public place and you make significant reasonable efforts to avoid anyone seeing you, that is to avoid getting caught, that you might consider that an ethical way to have sex in public. It’s still illegal, so I can’t recommend it, but from the perspective I’ve offered above, it may be more ethical.
