The reason it is in cultures/groups is because most people identify being Lesbian, Gay, Bisexual, or Transgender as a lifestyle, which ties in with culture. Also, a main purpose of the LGBT questions are about culture - from present day LGBT artists, celebrities, public figures, athletes, and politicians, to historical figures in LGBT culture, to LGBT political movements, to ironic stereotypical links to LGBT people, to figures and identities in LGBT culture (ex. : drag queens, pride, rainbows), and even LGBT groups.
However, I personally believe that LGBT sections belong in both the Cultures/Groups and Family/Relationships sections. Perhaps there should be one section for LGBT Culture and one for LGBT Relationships, because they are two veryyy different categories!