For me, I don't like slash, so I get more annoyed when something labeled gen turns out to be mild (or not so mild) slash. But I don't like thinking something's gen and then finding graphic het sex in it either. (But I do realize that an awful lot of SGA fic is slash, which is why I don't read a whole lot in that fandom.) There really ought to be more gen in all fandoms, though. When I first got into fanfic, that's all I wrote and read for about five years. I don't know why so many people seem to think there must be a pairing to make a story.