It depends. Certainly, playing online games with your partner, and "doing sex talk" either via the online interface, or via more traditional means (speaking across the room/down the hall, over the phone, or for slower online games, via postal mail exchange) is perfectly acceptable.
Assuming you mean "doing sex talk" outside your relationship, with someone you know only online… That's a bit more complicated, and perhaps impossible to definitively answer. Rather, if it feels like cheating to you, and you aren't willing or able to clarify it with your partner, then it's cheating. Cheating is not a specific act, a threshold of intent, or anything similar. It's simply "breaking the rules of the game".
Hopefully you're playing a game where everyone involved has read all the rules – and in the best case, where the rules allow for all manner of fun sensual expression, including "doing sex talk", at least with each other.
(Photo illicitly re-posted from the once-fabulous achewood.com)