I think there's several intertwining things happening here:
Girls and young women - completely understandably - are wanting to remove themselves from girlhood/womanhood, because girls and women get such shit in life, and during puberty this is a real shocker. They also see how transwomen are validated and celebrated, and their wants and needs met, in a way that actual girls and women's are not. Being trans from this perspective looks awfully attractive.
Girls and young women are also being told that they can genuinely, really, truly change sex. That they will be loved and affirmed for being their "true" male selves, and everyone will celebrate them as actual men - and since actual men are celebrated all the fucking time, this is a very attractive proposition.
The third, related thing: girls and young women who hang out on places like tumblr (where they will have it constantly affirmed that it is indeed possible to change sex) may well also be reading slash (m/m) fanfic. Now, I'm not knocking slashfic at all - I really enjoy the thing of reading, at one remove, romances where I don't have to think about the gendered expectations put on women in relationships - but they are idealised, mostly are written by women and not by gay men, and they are fiction. But if I was very, very young, and was convinced I could really, truly become a man, I would find these fantasy depictions of wonderful gay male relationships - where of course the heroes could never be so mean and transphobic to differentiate between men who have dicks and men who don't - very attractive indeed.
Hence straight girls and young women falling into the trap that gay men will accept them as gay men too.
Gay men, being men and therefore not having a lifetime of "be nice, put your own needs last" drummed into them, make no bones about wanting dick and being completely uninterested in female bodies.
Cue all this anguish from these young straight women who have had their castles in the air shot down.