For me, it depends... If you're doing it as a joke or if it's some RPG thing (many actors and actresses have pretend sex with others and still have SO or at least non-cheating breakups), I don't have a problem.
When I'm talking about RPGing, I'm talking about those people who pretend they're various characters, either already developed ones (ie. Star Wars) or some other fantasyland (ie. wizards, witches, superheroes).
But I want in on it or at least the option to read it if you're doing it. And the online sex better damn well be with strangers and no relationship outside the RPG (in that situation) better develop outside the cybering.
But that's just me.