something something the idea that women are purer and that love between women is more wholesome and pure and good leads to the idea that bi+ women in relationships with men are impure and wrong and betraying women and also the idea that there's something tainted about trans men for this.
it also is why i think a lot of terfs treat trans women like they're ruining and appropriating womanhood. it's the association they have with maleness being evil and ruining something good and perfect(femaleness), hence why trans women are seen as invaders faking womanhood for nefarious purposes. if maleness is tainted then trans men/trans women/bi women might bring this taintedness to female groups!