I have to question the universality of this nodetitle's assumptions. In my experience (and admittedly mine is limited, as is anyone’s) there are at least many women who do find gay men a turn on, though perhaps not in the same way as men are turned on by visual portrayals of sex between women.
Examples of women writers who have drawn erotically tinged portraits of gay men, whose readerships tend to be predominantly female would include Anne Rice and Mary Renault. (This particular phenomenon is considered in greater detail elsewhere.)
Among the complications I see in trying to approach this question are many issues relating to the "closet." After all, there are many examples one can name of gay actors who played (supposedly) straight heartthrobs, perhaps the most ironic being Rock Hudson in all those Doris Day films. Sixties and Seventies television is also rich in these examples, from Robert Reed in The Brady Bunch to Dick Sargeant in Bewitched.
Granted, in these cases the gay actor is, at least superficially, playing a "straight" role as husband or potential mate. But did such straight men exist outside of movies and television? I don’t recall seeing anyone act this way outside of the screen fantasy world.
This may be a highly subjective interpretation, but I know I grew up thinking that these fantasies must have something to do with the common experience I noted among my girlfriends of "falling for" unavailable men who were either unequivocally gay or who were widely assumed to be so. Perhaps this is something that has changed markedly since I was in my teens and twenties, given that these days it is sometimes more self-evident who is and is not unavailable — gay males are less likely than 20 years ago to pretend to be straight? Or maybe I just had very different assumptions than most?
This all may also relate to differences in what turns on women as compared to men? As others have noted, it may be more prevalent that women’s attractions are tempered by a more subtle combination of physical and emotional factors, with greater weight given to intangibles and to factors that do not reduce to purely visual cues. My sense is that much of the erotically tinged material aimed at women takes the form of romance novels and related fiction. While some is very explicit, much is less so, and rarely have I come across material aimed at women that feels as clinical, as focussed particular acts or body parts as seems often to be the case in porn aimed at a male audience. Most material that seems to be "made for women" assumes that there needs to be at least some emotional connection (even if sometimes a very negatively charged one) before going on to a description of lascivious acts and heaving flesh.
This is not to say that women aren't turned on by visuals (and I've had conversations with girlfriends who are turned on by gay porn.) But visual entertainment of the sort assumed to appeal to women is often more suggestive, less explicit, whether or not there is a real correlation between what is aimed at them and what turns them on.
This may have much more to do with the producers' prejudices and assumptions about what their audiences want to see, however, than with any real differences in what an average woman or man wants to see.
Whatever the cause, there certainly seems to be an assumption where "couples" films are concerned, where the typical differences tend to lead to films that are more about plot, and less about an anatomy lesson. I know that I personally find films that seem to be a visual catalog of positions and body parts not so much offensive as just a bit boring.
Getting back to the point of the node: How one gets turned on has a lot to do with what sort of images are going to appeal. It's not safe to assume — even if one could prove conclusively that women are not consumers of gay porn — that straight women are not attracted to gay men, or do not at least have fantasies that involve some sort of interaction with gay men.
Perhaps many of those Doris Day films contain a subtext of "seducing" the unavailable (gay) male, the male unwilling to be "domesticated" — these men, in the 40s and 50s were discretely and ambiguously presented as the "confirmed bachelor" and not necessarily (or at least not explicitly... many viewers may well have read between the lines) as "gay."
Maybe this also relates to why "lesbian" sex in male-oriented porn has little resemblance to life and love between women, since it usually seems to be mostly about "converting" those women to an attraction to men? Like much material where one can see a "gay subtext" but which was clearly aimed at a straight and often feminine audience, the typical "girl-girl" scene in male-centric porn is a fantasy of lesbian sex that is displayed to conform to male fantasies, and holds little interest (at least from what I've ever heard) for most women, whether lesbian, bi or straight. (Views to the contrary are more than welcome, though, I'm always open to fascinating exceptions or a good counterinterpretation.)
I have seen some pretty scathing comments from people (who represented themselves as men) about the few videos I've seen that actually portray something resembling lesbian sex as I understand it. Like films made for a gay audience, bona fide lesbian films do not seem to appeal nearly as well to straight men as do those fantasy "girl-girl" scenes that appear in many films aimed at straight male audiences.
Maybe the real question here is Why is there little erotica available that portrays common women's fantasies about gay men?