Well the answer the original question, for myself, I had a friend who was kicked out of their home because of being gay. That started me on my quest to find out what it is to be gay, well as best that I could being straight that is. I did some online searches when I had some time off and found Nifty. And I loved the stories. For me it started as support, but I don't know, I think it is hot to read about guys being together. Also, some of the stories are a really good read. For straight "erotica" or "romance" novels, there are only so many times that you can read about some studly well hung man saving some unspoiled pouty mouthed too beautiful beyond description damsel.
I also read HP slash fanfiction, and have come across some stories by guys who very much claim to be straight. Though a majority of slash is written by fangirls. The impression I get from them is that there is a desire to be the filling of a Harry/Draco cookie, if some of the comments are to be believed. Fantasy fulfillment, I guess.