Salon has an interview with two researchers who studied internet sex searches and organized them into usable data. And they've found more evidence to support what the average straight pornographer already knows:
There's pretty overwhelming evidence for it: Men search for penises almost as often as they search for vaginas. There's about a thousand heterosexual sites dedicated to large penises, and large penises are a category of erotica in all the major porn video sites. We think the reason for this is because it's actually from our evolutionary heritage. If you look at all the other primates, the penis is a prominent and versatile social tool used for many different purposes -- to indicate aggression, to mark territory, to indicated sexual interest. The penis may also function as a sperm competition cue. That is a very specific hard-wired cue for men to trigger arousal -- basically if a man has a reason to think that a woman recently had sex with a man it makes him more aroused and the penis must play a role in that.
Their book is on my to read list, but it'll take me a while to get there. Meanwhile, I wonder what they found about straight men's consumption of multiple-women-one-man porn vs. multiple-men-one-woman porn. Something tells me that the so-called ultimate straight male fantasy is, in fact, the penultimate straight male fantasy.