because they want to see lots of those they lust after
The same is true for heterosexual women
They like to look at hot naked men fucking,
and it doesn't really matter that they're fucking each other

The phenomenon of straight women who love gay male porn has been documented and was discussed plenty when flocks of females gushed over Brokeback Mountain.
When women came out of their fag-loving closet, it illustrated the flip side of a common theory: Plenty of straight men love girl/girl porn because they want to see lots of who they lust after.
The same is true for het women: They like to look at hot naked men fucking, and it doesn't really matter that they're fucking each other.
I think it may also be true that while pornos full of romance and mood lighting are marketed to women, some prefer a sweaty, get-right-down-to-it romp in the locker room any day.
Wired.com columnist Regina Lynn, author of "Sexier Sex: Lessons from the Brave New Sexual Frontier", writes that it's not always about identifying with someone in a scene:
"For me, gay porn has always been arousing because of its masculinity. The strength and power, plus the double dose of raw male drive and sexuality, add up to more than the sum of their parts."
On Nerve.com, journalist Kera Bolonik admits that she gets off with men onscreen, but not in real life:
"Nothing makes me hotter than watching two men going at it. There is something really admirable about gay male porn, at least in principle: it's egalitarian.
"Everyone gets a turn at the top as well as at the bottom. Everyone comes, and often they do it together."
I'm not sure this concept of equality is attractive to everyone (I personally like a hefty dose of power dynamics with my sex), but it supports the notion of seeing some kind of sameness, something non-heterosexual represented.
And it also raises the issue that many women, regardless of their orientation, have with porn: their ambivalence or discomfort with seeing female performers when issues of consent, enjoyment, and pleasure are unclear.
Somehow, our concerns and feminist politics don't overshadow the sex when women are absent (as if a man could not degrade or coerce another man, which is obviously flawed logic).