I never got the impression that Toast-era Mal was particularly fag-haggy, and she certainly wasn't a weeb, so what weird fantasy of gay men is she trying to achieve?
Her troon "wife" has a girlfriend, a second relationship, a full blown baby mama, while masculine manly Mal is just out for "sex with men." Not a boyfriend, not even a boytoy or a sidepiece. Just an act, "sex with men." Points for faking one obvious aspect of the gay male mindset semi-accurately, but it's a piece most pooners only pretend at while girlishly whining about why they never get called again afterwards. Does she currently have other pooner friends that she's trying to impress by being a real-live bathhouse slut?
But apart from that, as you say, writing homoerotic fanfiction (called "slash") is also as old as dirt and nobody pooned out over it until recently.
The earliest example of the slash-to-pooner pipeline is probably
Poppy Z. Brite, who was lucky enough to write a homoerotic vampire novel while being from New Orleans in 1992 in the immediate wake of Anne Rice, spent years claiming to be male-in-the-soul, and has been on hormones etc since 2010. While that was decades before the current wave of pooners, Brite was quite online by '90s standards and connected fairly early with mostly-HSTS trannies. I would
love to find an even earlier example, but the pre-internet norm of "going stealth" with trans shit and pseudonyms for fandom shit makes thst unlikely.
I was never a dedicated Toast reader, but it was unavoidable for a few years if you were following certain book and publishing scenes. (Which can be great to lurk if you enjoy drama too petty even for KiwiFarms.) I never got a notable hint of Mal being into slash nor any gossip about her having a not-so-secret AO3 handle of any flavor. Mal's arc would be less unnerving if it was as simple as "bookish nerd girl both fascinated and appalled by emotionally charged social interaction falls down the slash/MM rabbit hole, poons out" but I think it was a lot more about wanting to impress whoever she picked up that "mere heterosexuality" line of thought from.