So, does this make Giles being bisexual (we only ever see him romantically involved with women in the show) an official part of Buffy canon? Yes and no: as this history with Ethan is never explicated in either the show or the official comics, it is not technically part of Buffy’s sprawling canon.
At the same time, Espenson was a longtime writer and co-executive producer on the show who worked very closely with showrunner Joss Whedon, so her thoughts about these characters carry far more weight than, say some random fan theory.
Incidentally, even though Giles and Ethan being romantically involved is never directly mentioned, such a plot would arguably be fully in the spirit of Buffy, a show that frequently used the supernatural as a metaphor for the more mundane realities of our own world.
