I have question because I genuinely don't understand this. Why do you mean by transmasc sapphics? How come they be sapphics? Shouldn't they be straight? They are men aren't they? Sapphic is a term for women
great question! i don't think i'm licensed to explain this the best ever because i haven't had this experience but i will do my best.
first of all, i do want to point out that not all people who identify with "transmasc" as a label are trans men.
second of all, sapphic has long been used as an all encompassing term that doesn't only include women. for example, many genderqueer and non binary people are under the sapphic umbrella.
i'm assuming you might be familiar with the term "lesboy"? i've seen this one describe a lot of lesbian trans men and mascs.
a lot of trans men, prior to the use of that label, were part of the lesbian community, often as butches, or maybe also with the use of he/him pronouns. there a lot of butch transmasc folks.
a person who identifies with both "lesbian" and "transmasc/trans man" tends to have a more complex relationship with gender than say, a cis man who tries to call themselves a lesbian. "butch" as a gender identity is very complex and has a lot of history in the queer community.
all in all, i think there is a lot of nuance here. a lot of people will firmly say that trans men can't be lesbians, and a lot of people will just say they can if they do so respectfully and respect others' opinions on who they want to date and who they do not. it's all about personal preference sometimes.
in the topic of who i let into the community, i just try to avoid your average cis man. i'll generally let in genderqueer people, i don't really care that much.
i hope this helped some! i'd encourage you to do some more research about this topic because i am not a lesbian nor a butch nor a transmasc nor a trans man. so there is probably someone who would be willing to explain their relationship with gender that might make more sense.