I believe a large number of yaoi fans and readers ARE heterosexual, though.it's just that a heterosexual game developer making games for other heterosexuals is just going to naturally lean towards and ultimately feel more informed and comfortable towards heterosexual perspectives
However, remembering how my gay male friends fell over laughing after reading some of the slashfic I enjoy (Not stuff I'd written. I've never actually written slash. Yet.) does make me feel a bit nervous about approaching the subject, given my lack of direct experience with being male. For that matter, I've read some yaoi that had me screaming "BIOLOGY DOES NOT WORK THAT WAY!!!" at it.
The next chapter of Magical Diary will definitely have BxB options. Whether anyone but me will find them acceptable, I don't know yet.
There are BL forums. There are big slash communities. They're just not HERE, at the moment.