Because they want the validation.

If a lesbian is willing to date a trans woman, then it means that a trans woman is a woman. However, if a lesbian says "no, I will not date trans women, categorically", then it means that trans women are not real women. Which they're not. But they want the affirmation that they are biologically female, because it is homosexuality. Hence all the bullshit and coercion about girldick.

It may be that it's a small portion who are actually browbeating lesbians and gay men, but so many of them (even the ones happy to date bisexuals) go along with it anyway because to not go along with it also implicitly invalidates them.

That's all this boils down to. Validate me.

Edit: Also, it's not like the lesbian community is massive to begin with, and I think there's evidence enough that lesbian spaces have been completely invaded by "transbians". We may not know the actual number of trans women who are actively doing this invasion, but clearly it's enough to be a huge problem. Lesbians are not allowed to be lesbians anymore.