Disclaimer: I did not write what follows because I am prudish. I wrote it from personal flirting experience, watching other people flirt, and witnessing the consequences. All of what follows comes from, as they say, a practicing professional in the field of flirting!
Recently, I came across a guy who claimed to sincerely believe that flirting is not sexual. I have heard a lot of people use the phrase "innocent flirting", to mean flirting with no sexual intention/connotation.
What the heck is innocent flirting? The kind that you indulge in with someone of the same gender (assuming you are heterosexual) or the other gender (if you are homosexual)? The kind you would indulge in with a sibling or a close blood-relative? The kind you indulge in with a person whom you intensely dislike? Do you understand where I am going with this?
My point is that, we usually choose to flirt with the kind of people who are sexually oriented to us. By "people who are sexually oriented to us", I mean people who have 60% or more of the qualities which would allow us to have sex with them. We only flirt with the opposite gender if we are heterosexual, we flirt with the same gender if we are homosexual; we rarely flirt with people who are younger than or older than the age window within which we prefer to have sex; we almost never flirt with people whom we don't find attractive.
Gender, age and attraction are some of the most important criteria we use to choose the people with whom we have sex. If flirting was not about sex, why would there be very similar criteria for people whom we choose to just flirt with and people whom we choose to fantasize about/sleep with?
Even the very language of flirting is romantic and/or sexual. It may range all the way from being very obliquely romantic/sexual to blatantly romantic/sexual. But the romance and/or the sex is always there in the language. This is true of even the body language. The flirt and the "flirtee" display similar body language as a person who is being romantic/sexual with her/his lover. Even when the flirtee is not interested or dislikes the flirt, her/his reaction to the flirt is similar to her/his reaction to an unwanted suitor.
I don't think flirting is ever devoid of romance and sexuality. It may be true that the flirt and/or the flirtee have/has no conscious intention of taking things beyond verbal flirting. It may be that they don't really want to get physical. But I don't think the mind is truly devoid of the romance and the sexuality.
If someone thinks that they are flirting, and there are absolutely no sexual thoughts at any level in their mind, they are either deluding themselves or they are not flirting at all!
Flirting is the foreplay of the mind.
If it is true that sex is more in the mind, than in the body, then flirting is a much more significant and important act of foreplay than the actual physical foreplay.
Applying the Pareto Principle (or the 80-20 Rule): If one needs to score 100 points to have sexual intercourse with someone, 80 or more points are scored during mental foreplay (flirting), and only 20 or less points are scored during physical foreplay.
Flirting is almost purely about verbal communication. When future historians analyze the catalysts for the failure of the institution of marriage, an institution which seemed to control majority of the human society during a brief period of human history, they will conclude that the advancement in communication technologies (telephony, Internet, who knows what else will come in the future) was the main catalyst which broke the institution of marriage. Why? Advanced communication technologies enable mental foreplay, i.e. flirting, like never before! Naturally, this leads to more non-marital sex, than ever before!