Maybe the reason people find the lesbian label too restricting is out of a combination of:
It being the only sexual identity to the complete exclusion of men as both the subject and object of desire, and most people (even lesbians ourselves!) aren't used to anything that completely excludes men, much less on the realm of sexuality.
Lesbians being the smallest sexual identity group, so naturally most people don't relate to how attraction works for us.
If you, not being a lesbian, try to force yourself into the lesbian label, you're gonna feel restricted, same as any bi or gay person would feel trying to make themselves fit the straight label. Do you understand that whether a label feels rigid or not is entirely subjective? It WILL feel restrictive if it's not the right one for you.
I, being a lesbian, did in actual real life feel desperately restricted and suffocated by the bisexual label as well as the straight label, when I tried to force myself into each of them. I was always performing and policing my unattraction to men and punishing myself when I found myself not liking men "enough" (at all).
I didn't feel unrestricted when I identified as bi, I felt trapped and constricted and that's not the fault of the bi label itself and much less was it a sign that the bi label needs redefining, it was a sign that I wasn't bisexual.
That's what we mean when we say if the lesbian label feels too rigid or restrictive to you that might be because you're just not a lesbian, or you might have a lot of internalized lesbophobia (I definitely had the idea that "not giving men a chance" was mean/uptight/rigid/cruel so I didn't want to be a lesbian in part because I didn't want to be a Mean Dyke to the Poor Men who might desire me and thus had a "right" to a chance with me).
It's not an insult at least when I say it. It's just a fact.
