Ugh. Please don't bring this redpill malarkey to hackernews

In this hypothetical example are you walking over and saying good morning to each of your male coworkers as well?

The point is, treat your coworkers the same, regardless of gender.

I haven't been in the workforce very long, so I really don't know and am asking genuinely. Is it ever okay to flirt with a coworker?

Yes, but the problem is that in most of the scenarios that get discussed it's not flirting but one person making advances and not noticing (or caring) that the person is actively feeling uncomfortable.

The rule of thumb is sort of in line with telling a risque joke. Is that ever OK in a workplace? Sure, but if there's any doubt whatsoever how it will be received by the audience then you probably shouldn't be doing it.

I was sorta with you at first, but that second paragraph is straight-up nonsense.

I've seen it happen, though it sounds like someone is over exaggerating women's reaction to their ugly mug.

But yeah, I knew a woman that said anyone who smiled at her was creepy if she didn't like them they exist.

No one cares about a purist stance on creepiness. Being creepy is enough to justify removal from any social or professional situation. If you lack the social skills to avoid being perceived as creepy, that's a you problem. It doesn't really matter what your rationalization is. People aren't going to want you around.