In simpler times we were all gay. But then the word "gay" started to mean "gay men" more than women, so we switched to the more inclusive "gay and lesbian." Bisexuals, who were only part-time gays, insisted that we add them too, so we did (not without some protest), and by the early 1990s we were the lesbian, gay and bisexual, or LGB community. Sometime in the late '90s, a few gay rights groups and activists started using a new acronym, LGBT -- adding T for transgender/transsexual. And that's when today's trouble started.
Heh. It's a bit like the flag. "Gay" was just about an umbrella term covering everyone, but individual groups wanted attention. The rainbow covered everyone, but individual groups wanted attention.
Another minor point - "gay" is an adjective. "Lesbian" is a noun. Both of them have crept to be used as either, but just from grammatical correctness, a woman would use gay when an adjective was needed.
Another example - Basic Instinct (1991). Beth: "What was I supposed to say? 'Hey, guys, I'm not gay, but I did fuck your suspect!?"'
Took a little while for "lesbian" to be accepted as an adjective. Is the tautology "lesbian woman" acceptable yet? I'm not finding it horribly grating, at least, which means everyone else probably is using it happily.