Board Thread:General Discussion/@comment-27907368-20160323193924/@comment-27620479-20160704170220

Not to mention that Alphy's date isn't even real, because Frisk doesn't initiate it, they just deliver Undyne's unsigned letter, and Alphys isn't serious about it either, as she admits that her intention was a pretend date.

By flirting/dating logic, Frisk could be a boy, as they can flirt with Toriel, hang out with Papyrus, flirt with Tsunderplane, hit on Undyne, and don't mind dating with Alphys, who seems serious about it.

Or they could be a girl, as they can date with Papyrus.

Or just seriously messed up, as they can flirt over the generation gap, with one or more different species, some of which aren't even vaguely humanoid.