well, which is more important to you? Her or the porn? Sometimes you have to make sacrifices for love, especially if something hurts the other partner. Know that she feels it's a replacement for her and she is very threatned and hurt by it. If she says it's cheating to her, then that's quite a painful thing for her to experience.

Are you watching it, or is she just upset that you think it's okay? There could be a huge difference there.