Is lesbianism a 'thing,' or a passing fad?
I'm curious why you ask. If a woman is using it as a non confrontational way to decline a date with you, well, then it might be a tad bit of a fad.
Aside from that, it's a biologically natural expression that exists in many species, and I've never known a Lesbian that reverts to heterosexuality, so I'm inclined to think that, yeah, it's a real thing.
The first usage of the word "lesbians" was for those who did not believe in traditional marriage. That was a union where women essentially had the same rights as children, none, could not own property, did not have rights over her own body, could not go anywhere or do anything without permission of her husband, and could be legally beaten. I hope everyone here is that lesbian.