The reason some "lesbian" women sleep with men is because they aren't really lesbian. They're bisexual but just say, either to others or to themselves as well, that they're lesbian.
The reason they publicly call themselves lesbian is because women who identify as bisexual are usually very slut-shamed because they're, you know, obviously massive sex addicts that want to bang everyone. After all, bi people do have wider selection than straight or gay people. And who wants to be called a slut?
There's also the fact many actual lesbians are wary of, or even hostile to, bi women. Mostly it's because they think bi girls will just immediately jump onto the cock of any man they meet. That and the fact that they think bi girls are actually just straight girls who are just looking for attention from men by making out with other girls when they're drunk. So in order to not attract hostility from lesbians, they call themselves lesbian.
However, many lesbians do like dicks. After all, why else would they get themselves dildos? They just don't really fancy the man around the dick.