There's this line that I can't quite remember. I think it's something like, 'I don't know what porn is, but I know it when I see it.' Something like that. Granted, porn is a poor thing to bring into this discussion, because I don't think that flirting and porn are related at all, but by putting the two in the same post...well.
That's how I think about flirting, though. If you asked me to define what constitutes flirting, I couldn't tell you - not in any specific way, at least. But I know it when I see it. Or when I'm doing it. Anything can be a flirt. It (like so very many other things), depends on the person (people) involved. Everyone finds different things attractive, entertaining, interesting. So something that is flirtatious to one may not be to another. Humanity is not a monolith!
I've flirted over comics. And I've had perfectly platonic conversations over comics. Very similar conversations, actually. So what made one flirting and the other not? The people involved. There's a start to a conversation, an interaction, where it's simply two people talking. And then there's a shift and your voices change, subtly, your postures, and both (if both parties find the other interesting/attractive) recognize that the conversation has an undercurrent.
Now, does this have to go anywhere inappropriate?
First, what's 'inappropriate'?
I would think that we can agree (most of us), that inappropriate would be sex, or anything that immediately arouses and/or leads up to it. So, no heavy petting, no making out, no groping, no second, third, fourth, home run whatever. This is because sex is something that should only happen between a married couple. And anything leading up to sex, I think, falls into that category automatically.
But flirting is none of those things, in and of itself. Flirting is like the opening salvo. If you don't find someone with whom you click, with whom you can have this easy exchange, then you shouldn't date them. And dating is what leads to marriage. Of course, I know of at least one person who doesn't believe that Christians should date, (hi caraboska!), but I disagree. So long as both parties are respected, and the lines are clearly drawn as to what activities are acceptable between two individuals as opposed to those that are acceptable between a married couple (wherein they have become one person), and both individuals are respecting of God, then I don't see dating as a problem.
As a matter of fact I think remembering that God (and all the angels, and the saints, and all your departed friends and relatives) are *watching you* would help keep any stray libidinous thoughts under control. That's right, your great aunt Frieda sees what you're doing there!
Now, the Bible doesn't deal specifically with 'dating', and I guess some people argue from that that dating is not a Christian/Biblical thing, but since the concept of 'dating' or even choosing your own spouse based off of mutual attraction and love didn't really fly back then, I don't think you can use that. We have to take Christian concepts, and apply them to the times that we are living in - not attempt to live as though we were living 2000 years ago.
So what is forbidden between unmarried individuals - sex and lust. I guess we can argue as to *why* those are forbidden. For instance, is sex forbidden between unmarried individuals because of the need to be able (on a material level) to trace the lineage and for men to be certain that the children they are supporting are *theirs*, and that their honor (and, of course, that includes the honor of their wives) is intact. Which is a purely secular and 'manly' reasoning. Or because in marriage the individuals are meant to come together in a deeper way, the trust and respect between them should be as pure and as singular an event as can be expected and marriage reflects (as well as anything corporeal and created can), the love that God has for humanity and the community that the Trinity has amongst itself.
Lust is obviously forbidden because it leads to sex. And I suppose that to some people's minds, flirting must mean that there is lust there. But attraction is not all about (or even always about) lust. We are attracted to people without *lusting* for them all the time.