I just realized that the form of flirtation that's harmless fun that won't go anywhere is romantic role-playing. You've got a Mary Sue or Gary Stu character, an idealized form of yourself, and your character goes and interacts with romance-tinged wit with all the other Pretty Characters out there. At the end of the session, no harm, no foul, and maybe you have some new friends. All parties (should) know that it was just a game. This is mock courtship in its courtly form.
There's a kind of malicious flirtation that I was victim of in middle school and the early bits of high school: where the one party puts out a Gary Stu to court the other person's real self, and then if the person's real self falls for the Gary Stu, the player mocks the duped party. Real courtship and mock courtship should only be mixed with compassion and caution.
Then there's the other side of mixing mock and real courtship: Adam courts in seriousness; Beth thinks it's mock courtship. Beth responds with a character. Adam falls for the character. Beth is flirting without intent to follow through in good faith; Adam is flirting with intent to follow through in good faith. Collision. Heartbreak. Possible skeeviness.