Social conventions are changing and the language changes. Words like "gay" begin as slang and colloquialisms and then are adopted into standard English. Meanings drift over time. In the U.S. in 2014 I believe that "Gay" has two DIFFERENT meanings. It can mean any kind of homosexual, male or female. For example, when speaking, same-sex marriage is usually referred to as "gay marriage" and includes both male and female couples. A few hundred years ago, it "gay" just meant sexually uninhibited or licentious, and could be applied to heterosexuals. That meaning is still in the dictionary but don't try to use it that way today! It used to mean that, but it doesn't any more. The meanings of words like "witch" and "wizard" have been drifting and changing definitions because of because of its use in fantasy novels, and also because of the new-age Wiccan movement. In a fantasy world, the writer makes the rules, and a "witch" and a "wizard" can be whatever they are in the writer's own world. Because wizards are usually thought of as being powerful, dignified, and often on the side of good, any modern feminist would insist on the (fantasy) profession of wizarding being open to females--just like veterinarians, corporation CEOs, or prime ministers are, in the real world. When I was a kid, I understood that male witches existed--in the world of superstition and the imagination--but that they were called "warlocks." That's probably not true today.

Forgot to mention the other meaning of "gay" in common U.S. parlance. As I said, sometimes it includes both male and female homosexuals. Other times, as in the initialism GLBT for "gay, lesbian, bisexual and transgendered," "gay" does mean "male homosexual," as opposed to "lesbian" for "female homosexual."

Males can be called wizards, witches, or warlocks. Females are usually only witches. Gay actually means happy but people use it to describe homosexuals. When using it that way gay and lesbian mean the same thing.

I don't watch those shows, but I can help a little. The differences in witches and wizards are normally up to the writer, since those things are fictional. Normally, as you stated, a witch is female and a wizard is male. Lesbian is used to describe women, but gay is used to describe either men or women.