Because he's holding a book, I mean. The existence of this blog, Hot Guys Reading Books, suggests that many women might indeed want to bump on the basis of books alone.
The site may implicitly endorse the idea that women are attracted to softer and fuzzier qualities or the stories around things, while men simply want tits and ass (The New Yorker's Book Bench blog notes the Liz Lemon-esque dimensions of the concept). But there's clearly something going on here, even as a niche taste. (And of course, niche is what Internet eye candy is all about.) And the author does spend plenty of time evaluating the physiques of the subjects, in addition to the titles of their books.
Throughout Hot Guys Reading Books, the creator begs for submissions and gripes that she has resorted to taking her own photos. That may be because people find it creepy, or it may be for the same reason that anyone might find the site titillating: a guy reading is seen as being like a unicorn.
After a recently reignited debate about women, men, and reading — centered on whether alleged female domination of book publishing was driving men away from the bookshelves — Amanda Marcotte pointed out on Slate's Double X blog,
Reading fits the socialization patterns of women more than men. Boys are brought up from a young age not only to believe that reading is for sissies, but also that their desires and entertainments can be obtrusive and uninterrupted. Girls are brought up to be bookworms, and, in part, it's because it's a quiet past-time. The stigma attached to reading falls away for men as they grow up, but they don't stop feeling entitled to entertainment that is loud and engrossing enough that it can't simply be dropped because a chore has to be done or a family member needs attention.
So maybe, slightly perversely (given history), getting hot and bothered over a guy reading is just about liking his subversion of gender norms.
Hot Guys Reading Books [Tumblr]
These Are A Few Of My Favorite Things [New Yorker/Book Bench]
Why Do Women Read So Much? [Slate/XX]
Why Men Don't Read Books [Salon]