Probably the single biggest factor in the discrepancy is how secure a woman feels in the relationship. The more secure she is, the less likely she is to let it bother her. Of course, there are other factors, such as why/how/how often the man uses it. Some women would rather their guy watch it with them, and it's a problem when they do it alone. A woman's perception of what it is (is it erotica or female exploitation?) and also the man's history with it can be taken into account (used to be an addict?).
In my experience, here are some reasons why women don't like it:
- feel like the man is looking at (and therefore wanting) better looking and/or more experienced/open women than her
- man is neglecting her (emotionally and/or sexually)
- afraid the man is going to want to try some of that freaky stuff he's been looking at
- sick of finding the evidence of a "session" around her computer

- afraid the kids are going to come across it and/or him using it