Well not only realizing at the various ways to men can enjoy each other, but the idea that there can always only be one "pitcher" and one "receiver" (which actually is very close to the translations of the original Japanese words used for these "roles"). That's sometimes the case, but not always, and not even necessarily most of the time. Part of this is that BL was originally written by women for women, and this plays right into lack of knowledge you're talking about. They forced an oftentimes extremely questionable heteronormativity into gay male relationships, and once it started to become extremely popular translating to a visual medium, it's been a slow evolution away from that heteronormativity, to a point where it's starting to look bright at the end of the tunnel, but those fangirls sometimes can't get rid of their ignorance. Ask @Drew Espinosa about my various rants regarding that subject as well.