You may be perusing Literotica or another [lesser] erotica website. You may find a new lesbian erotica tale that you start reading. And you may say to yourself, hmm, this lesbian erotica seems odd, but I can't quite put my finger on it. Allow me to assist, Fearless Reader. For in fact, you are not reading lesbian erotica -- you're reading vampire erotica. In these stories, lesbianism is portrayed as something that can be spread like the fucking plague, or, like, you know ... vampirism.
Now, can lesbians be vampires, and vice versa? Of course. In fact, there's a whole subgenre built around precisely that. But there are also stories out there that seem to want you to believe that they're lesbian tales set in the real world, where in fact the real world is nowhere in sight, stories that only make sense if you accept supernatural elements in them, but that never admit being supernatural stories.
Some points to consider, if you will:
1. The initial sexual encounter almost always involves a heterosexual woman ("straight") who has never had sex with another woman before. The encounter is almost always portrayed as an initiation in which the straight woman is docile and/or submissive.