Um, I am happy for people to have non-private sex, but would prefer not to used for someone else's gratification without my consent.
So if I go to a place where non-private sex is likely to be happening, and I know that, then I am consenting to being used as a bystander or potential onlooker in someone's desire to have non-private sex, no big deal.
If someone is hoping to be private and I happen upon them by accident then, well, that's just unfortunate for all parties.
But if someone chooses a very public place for sex and 'the public' couldn't reasonably be expected to know or suspect that, then it's very selfish, at the least. And at worst abusive.