I've never engaged in "romantic" flirting partly because I've never dated, so didn't develop those skills, and because I was always too wary of men and their intentions. The flirty girls that I knew, were always involved in dramas and seemed to attract the wrong type of men, which further put me off.
Even though I only recently found out I'm on the spectrum, I've always felt vulnerable around men so my demeanour is always neutral in their presence.
As for understanding flirting, I kind of do... I know that many people seem to find it fun, but maybe I'm just too serious and uptight 