I don't think it is. It's your own fault for not indulging his fantasies. If he wants you to talk dirty with him, then do it! People don't go on sites like that because it's a hilarious Lolcat site, they go because they want what they're lacking. There's no emotional or physical contact so I don't consider it cheating. If he's faithful to you in every other way, you should reward him once in a while with some dirty talk. I guarantee you that if you do, he'll stop.