What are the ethical issues associated with studying people who may or may not know they are being observed? Sometimes the issues are ambiguous

Give an argument that the observation done in the research by Humphreys in The Tearoom Trade was ethical; then give an argument as to why it might be unethical.
What will be an ideal response?

If the behavior being observed is public behavior, there are few ethical concerns about studying those people engaging in those behaviors. If the behavior is not considered public (even if it occurs in a public location), there are ethical issues concerning invasion of privacy.
In the research by Humphreys, you could argue that the research was ethical because the men who engaged in the sexual behavior did so in a public area and with anybody who happened to be there. As such, you could say that they had no real expectation of privacy if they didn't care who participated with them.
On the other hand, you might argue that sexual activity, even if anonymous, is a fundamentally private act. Even though it took place in the rest room of a public park, the men engaged in the behavior were simply using a convenient area, but that the act was still a private act. So the researcher was invading their privacy.