The thing is, this not only effectively erases lesbians (as others have already said)-- it also erases bisexual women.

Because what you're describing here IS female bisexuality. Yes, even if the woman doesn't seek sex with men on a regular basis; even if she only occasionally finds a man attractive; even if she just sleeps with them for fun, without any sense of emotional attachment. "Bisexual" simply means being sexually attracted to both sexes; it doesn't matter whether that attraction is equal-- you can be decidedly more inclined one way than the other-- or involves liking/loving them. The term covers all of this.

But if it's included under "lesbian"... what does "bisexual" mean anymore? Particularly since it tends to be forgotten/ignored/dismissed as it is.

In order for both bisexuality and female homosexuality to be recognized and respected, "lesbian" must maintain its definition as "woman or girl who is exclusively same-sex attracted". Otherwise, we both get lost: lesbians are misinterpreted as bisexuals, and bisexuals just plain disappear. We've already seen what damage the former can do ("girldick", anyone?); I sure don't wanna see the latter come to pass, either!