Women tend to be aroused a lot more by the idea of things, rather than the visual stimulus. That's why they tend to picture themselves as the woman in porn.
When it comes to lad's mags though, it's an element of comparison, but well.. hmm. I kinda get a bit turned on when I'm looking a lingerie online etc (on girls, obviously - just on its own you don't get the same image formed), because I think of myself wearing it and the situation etc etc... similar kinda thing I expect.