First off, why would come to a thread about religion/spirituality and post a joke thread about masturbation?
Second off, in Christianity it is a sin, a worldly sin. When God created man and woman, he did create sex (unfortunately a taboo topic in most churches). I don't see any problem discussing sex in church, it's not dirty. I think when in the context of marriage, it is a beautiful wonderful thing (at least that's what I've heard). But it goes on up there with lust and it is a sexual act. So yes in Christianity it is a sin. Because you are pleasuring yourself.
And to rocketqueen, I think we could see your screenname and see that you have no problems with self service.