Derived from the Hispanic dessert served throughout
the southwest, flan is
sexual slang for a near-vanilla person. As you can be straight without being narrow-minded, you can be flan but not vanilla. The BDSM / fetish / leather world has often used vanilla to define someone who is not into any kind of kink. Often it has a negative connotation, so flan is a positive word for the same thing. They may like conventional sex themselves, but they support your right to enjoy yourself in a safe, sane,
consensual manner.