If you center the entire lesbian identity on "nonmen loving nonmen", you kinda paradoxically have to define what a man even is. And as you might guess from the wave of anti-trans legislation sweeping several governments, this is both a question that's very easy to politicize, and also difficult to coherently answer.
So like, let's assume we take the gender-identity respecting, broadminded answer of "a man is someone who in good faith identifies as such".
For one thing, it's odd that we're applying this broad idea of self-identification to the idea of "man", but not the idea of "lesbian".
(Almost like these two ideas don't quite go together and people who promote the "nonman" idea might perhaps have a different idea of how to define gender as it relates to a rigid sexual binary...)
But for another thing, using this definition of "man" to create the opposite in "nonman" used in our definition of "lesbian"? It's kind of a mess with shoddy reasoning and ample edge cases.
Like, often people pushing the nonman definition will acknowledge that several transmasculine identities can be included in lesbian attraction, so long as everyone involved (the lesbian and transmasc both) are okay with labeling their feelings thus --
(Again, rather odd that this openness shows up here, but draws the line at the mere concept of men, almost as if this definition is at best a retread of lesbian separatism, and at worst, outright TERF rhetoric coached in enough plausible deniability that even people who believe transgender people exist are getting fooled into using it).
-- but sure, good for them, respect for nonbinary identities.
But like, how do we separate "transmasc identity" from "man". We've got overlapping, fuzzy concepts here, and this is supposed to be a formal definition.
I've seen at least one person who tried to word this distinction as the idea that lesbian attraction does not include people who are "in any way men".
I'm not sure this actually solves the confusion here, but it did lead me to realize a huge edge-case in this definition.
Imagine a genderfluid person, who on some days, considers themselves a man, and on others days considers themselves a woman. That's a fairly vanilla, platonic ideal of genderfluidity, I feel.
But you see the problem, right?
Either this person can only be a lesbian on certain days, or the fact that they're sometimes a man means they cannot be a lesbian at all. Either solution here is completely absurd. And the latter may be outright bigoted.
(Also, I invite you to think where you've heard the kind of gender framework where manhood is inherently corruptive, that sits like a stain on you even if you move away from being a man.
I'm not being subtle about this: this whole definition of lesbianism originates in transphobic movements).
If you've followed my chain of logic this far, your response to the genderfluid example might be that the answer should be between the two absurd conclusions. That it depends on the individuals involved, and their boundaries. Just like the transmasculine identities, some people would include this in their lesbianism, and some wouldn't.
And, uh, yes, that's exactly correct.
But that solution is also kind of antithetical to prescriptively defining lesbianism as something that has to be a certain way, rather than a broad category of things it often is.
Either you accept that things like "man", "transmasculine", or "nonbinary" are broad, vague categories that depend heavily on personal experience and expression.
Or you don't, and start echoing gender essentialist talking points.