What makes this unusual is that many yaoi characters do not seem to have any particular orientation--until they do. It's a world of demi-sexuals in which people are not in love . . . until they are! Love is so overwhelming and terror-inducing, everything falls before it.
Although Fake is rather unusual in its characterizations, it utilizes many charming yaoi tropes. The most important is that like Scully and Mulder from The X-Files, the main characters maintain a kind of holding pattern: not waffling but not completely committed either.
They are together, of course, a unit, and the reader is treated to many sweet and loving moments, but there's no need to worry about the big declaration: not until the last volume anyway.
The truly refreshing thing about yaoi isn't that it allows for diversity--or not. The truly refreshing thing about yaoi is that it doesn't seem to think that falling in love is anyone's business but that of the people in love. One doesn't have to take out a referendum on the topic. It's about the individuals, not society writ large. For once, politics--or for that matter, academic theoretical language--is not allowed to hold people ransom with inherently debasing and flattening rhetoric!
And yes, there is a new Fake series--which I will review at a later date--