My first slash experience was with Twilight. My daughter was a huge fan of the books and introduced me to the fan fiction. I started out pretty vanilla reading stories that came up on the Twilight message board and through the links to fan fiction.. . .I then went for a ride on the wild side and started reading slash Edward/Jasper stories. I got to be friends with an author who wrote a slash story that got to be a pretty big deal in the fandom so much so that she yanked the story, revamped it and has published it commercially --- anyway, once I crossed over to slash I stopped reading Twilight het (although a good Jasper/Bella story still holds appeal).
I got into H/S and H/D because some of the Twilight authors crossed over and haven't been back since. I haven't read any HP het. I don't know why although I think it has to do with the fact that I never got the Hermoine/Ron pairing or the Harry/Ginny pairings. I haven't read too many other HP slash pairings and I think it is because I just can't wrap my mind around HP/TR or Voldemort and can't see Severus with anybody but Harry (although Severus and Draco do make sense and I have read a couple of H/L stories that are pretty hot.)