When is it appropriate to bring up sex?

Ok, please don’t give me any of this “when it feels right” stuff, because it never feels right. A lot of my talk is about normal stuff (as if I am talking to a friend), so there is no wonder why most girls view me as a friend or “just there.” I’de hardly talk about it even with my ex’s (at the time). I feel this is what seperates bf\fb material from just there material. Since even my sex-ful relationships sucked I have nothing to base off of.

Anyway, when is it appropriate to bring up sex, expecially when talking to women online.

Now everything I read online says “don’t mention it” or “its rude” but from what I see in real life (friends and such), people text, online, and talk about it all the time. I’m getting conflicting sources of information and I do not know which one to believe.

I think Fluther can give me a “real life” sample, so Fluther, What to do?