Maybe she is insecure and a little jealous, but have you considered the possibility that you nudged her in that direction? There are women who don't like porn (obviously), but most of the time they accept that it's part of a man's life. But if it's interfering with your life with her, then it changes her perspective. She may be getting plenty sexually, but could there be other areas that make her feel inadequate in the relationship that you could be contributing to?
If that's the case, you even looking at a Playboy reminds her you're desiring another woman (even if it is just fantasizing for a few minutes), and if she's already feeling at all insecure in her relationship with you, this will only deepen that feeling. So of course she's going to want you to stop, though this only gets rid of a symptom and doesn't address the real problem.
I would just start looking at all areas of my relationship with her and see if there are any loose ends you should be working on. If not, maybe she is just a woman who has no self-esteem and needs to mature a bit
