they figure that it's a one-drop-rule sorta situation-- if a woman is attracted to her own sex at ALL, she obviously can't be straight, and therefore must be gay.

That's definitely what I was taught when I was younger by the straight people around me. The gay community has no influence over the greater culture (as we have seen over and over again in recent years.)

They feel that there's a moral obligation to "stand up and be counted" as lesbian.

This is a new one for me, but it does have a logic to it.

The explanation frequently given by people here, "they want to be special and steal our shine," makes no sense. Lesbians get treated like shit. Bisexual women know this.

I've always used the rule people don't just do things, there's always a reason. If you refuse to try to dig into why other people are doing something, in their own logic, you'll never figure out a way to convince them to stop. If they think lesbians are weak and need their help, the way to stop them would be to demonstrate that lesbians are fine on their own.