Crystalrose, I'm not Christian and this is a bit off-topic, but just to respond to your not understanding why Christians are generally against masturbation is because they relate it to the story of Onan, who was supposed to impregnate his sister-in-law after her husband died. It was his duty as a brother. Instead, he masturbated and "spilled his seed" and that was considered a sin. I don't ascribe to this belief, but that's where it comes from. It especially doesn't seem to apply to women, since we don't have "seed," but I hope that clears things up. It may not be the only reason some Christians are against it, but that's the general reason that's brought up.

As for porn, I think it's because Christians feel that kind of pleasure is meant to only be between a man and a woman privately, and not meant to be watched by others. Also, much porn doesn't depict a married couple (some does, though). There are a lot of reasons Christians are generally against porn, and I won't go into it here, because that's a long topic and this thread isn't about that, anyway.