bespin, I'm afraid you're talking to a person who believes in absolutes. I do indeed accept that there are women who hold the philosophical belief that transwomen are women and therefore define themselves as lesbians if they sleep with a transperson with a penis.
They are wrong. The protected characteristic and therefore the legal definition of 'lesbian' is a woman attracted exclusively to her own sex. Transwomen are not of the female sex. Therefore they are not lesbians, however much pleading, coercing, wheedling or wordplay happens.