The primary audience of the 30,000+ Slash (that's gay) Harry Potter Fanfictions are female. The vast majority of my audience for that story, are in fact female.
Females seem to have the same interest in two boys going at it that most boys have about two girls...
As for straight guys? I don't think much do. I'll tell you this though: I have a large number of straight friends, all classmates and smart people... I don't think they every read much of anything for enjoyment.
Bi guys have contacted me and seemed to have shifted to more gay after seeing gay romance portrayed. (I've pointed them to different stories from other people to see what they thought).