I'd be happy to explain. First a warning: I must reference some extremely hateful language below; this is not my language, it is the language of people calling themselves "gender critical feminists."
The most famous, long-standing communitiy on the internet who call themselves "gender critical," is probably the reddit "r/GenderCritical." Here's a snapshot from that "lovely" place
There's a lot going on here. "MTT" is what these people call trans women. An old term for trans women was "MTF," meaning "male to female." By "MTT" they mean they refuse to see trans women as either male or female; they continue this pointlessly demeaning language by using "he" pronouns just because their M.O. is to be as insulting as possible. If someone tries using more sympathetic language there they get shot down -- because sympathetic language, may, in fact, engender sympathy.
They are so confused in their gender theory that they call these trans lesbians married to each other, "straight men." "Autogynephilia" here is their pet conspiracy theory that trans women are perverted men turned on by the idea of being women. Because they deny trans experience, these self-described "gender critical feminists" have developed a whole system to try to explain trans people's motivations as anything but gender identity and physical dysphoria. So since they think trans lesbians are "really" straight men, they're left with the absurd conclusion that trans x trans lesbian relationships are "really" just two straight men.
The poster in this snapshot is so freaked out about these trans girls passing because one of the main "gender critical" talking points on that forum is how they can always tell when someone is trans -- which would be a prerequisite to building trans exclusive spaces as they seek to do -- this reaches absurd levels such as when they called this murderer a trans woman without evidence; she is just a cis woman who appears very masculine due to health and presentation.
This post isn't by a one-off individual, as the comments later on the thread show; they state here they believe all "mtts" want to "rape" lesbians.
These are typical words of the people who have claimed the "gender critical" words on reddit. Similar can be found on Tumblr or Twitter. This is phobic, conspiracy theory level stuff, like the words found in any other hate forum online. The words "gender critical" sound, at first, good: who wouldn't want to criticize gender? But these words above, aren't good words, and they don't, in any meaningful sense, criticize gender.
This is why "gender critical" is so effective as a dog whistle; it sounds nice from the outside, without revealing the rotten, hateful underbelly of the kind of language actually used on "r/GenderCritical" and other "gender critical" communities.