Why do we do genealogy? For most people, the answer is probably a mix of interest in family history and enjoying the research. There's also a social element, connecting with other interested relatives and collaborating with others on the research process.
Most genealogists focus on their direct lines, but researching the siblings and their children and grandchildren yields new clues and adds greatly to the family narrative. It's natural sometimes to get caught up in broader interests like surnames or locations too.
Most genealogists also have a healthy respect for the living, and reflect that respect in how they research, what information they choose to collect, and what information they choose to share with others or publish. However, some intentionally post information on living people without the permission of their subjects, violating expectations of privacy and the social norms of this endeavor.
Suppose a family submitted a birth announcement to a local newspaper before the advent of the Internet. Strictly speaking, that information is publicly known. But posting that information on the web today for the entire world to search and read violates the contextual assumptions that were in place when that family happily told their town of the birth. That's just one simple case.
But privacy isn't a simple concept, it's far from all or nothing, and it's not the same as secrecy. It's an essential human right guaranteed throughout the US Constitution and through other laws and agencies [e.g., 1, 2, 3, 4, 5, 6], as well as through numerous international laws and treaties throughout history.
The National Genealogical Society (NGS) features privacy quite extensively in their Standards for Sharing Information With Others:
1. respect the restrictions on sharing information that arise from the rights of another as an author, originator or compiler; as a living private person; or as a party to a mutual agreement.
6. require some evidence of consent before assuming that living people are agreeable to further sharing of information about themselves.
7. convey personal identifying information about living people—like age, home address, occupation or activities—only in ways that those concerned have expressly agreed to.
8. recognize that legal rights of privacy may limit the extent to which information from publicly available sources may be further used, disseminated or published.
and in their Guidelines for Publishing Web Pages on the Internet:
6. adhere to the NGS "Standards for Sharing Information with Others" regarding copyright, attribution, privacy, and the sharing of sensitive information.
11. respect the rights of others who do not wish information about themselves to be published, referenced or linked on a website.
Even the Board for Certification of Genealogists' Code of Ethics and Conduct prominently features client privacy in its professional pledge.
So please, even if you think some information is "public", respect the wishes and privacy of living people and get their explicit consent if you feel the need to collect or share their information.
Remember... genealogy is a hobby, the rest is real life.