Margaret Mead described "culture" as "an abstraction of the body of learned behavior which a group of people who share the same tradition transmit entire to their children, and, in part, to adult immigrants who become members of the society." See Wikiversity article.
This emphasis on culture being a learned behavior works well for those who believe that homosexuality is a choice or that gays who become parents are training their children to be gay or that all gays are pedophiles. The term "gay culture" often serves as a pejorative when used by members of the homophobic culture. James Savick has described the homophobic culture quite succinctly in his comments.
I remember the hippie culture being disparaged in the 1950s and 1960s as long-haired, unwashed nonconformists. The homophobe sees gays as something worse than nonconformist. Being considered a culture is a compliment for gays compared with other terms homphobes frequently use.
I agree with earlier comments about the multiple cultures to which we all belong and the term "gay culture" being a totally incomplete description of who we are. But then I wonder about my own cultural assignment. I am a straight, southern, Christian father and grandfather who has many gay friends and who speaks up in forums such as this. Am I a member of the gay culture? If I am, how does that affect the definition of "gay culture"?