When you make the center of something "not X," then X is still the center of that thing. The center of my lesbian... ness? Lesbianosity? Lesbianity? Whatever. The center of the quality that makes me a lesbian is my attraction to and love of women (and multigender people with women in the mix, and the infinite other subtle variations on the theme of woman existing in the realms of gender), not... not being attracted to men.
And the thing is, right, when you make a negative thing the center of something -- "not into men," in this case -- it makes that thing negative, and it makes that thing both fragile and brittle. There are so many things about Sapphic love and lesbian relationships which are wonderful in their own right, and not only don't require this whole "no boys allowed" mentality, but suffer from its inclusion.
When you make the center of a thing "not this thing here," that fragility created by that mindset means that the complexity of humanity threatens your identity. If your lesbianosity, the quality of you that makes you a lesbian, depends on not being attracted to men, what does it do to your ego when you experience the "happens often enough that we joke about it" of hitting on a twink at a gay bar? You know?
If your lesbianism centers your love of women, then, well, whatever. You can brush that off easily, because you're not defined by not being attracted to men. But if you are... well. Fragility.
And when you shatter, your edges are sharp, you hurt people.
Plus, ugh, it's just such a fucking drag to deal with people who define themselves by what they're not. It's exhausting. It's like talking to someone who is forever stuck at the part of their teenage years where they're trying to be everything their parents aren't. Like, we get it, your parents are boring squares, but who are you, really?