I actually think this is not a BL but more of a trans (transgender) love story. The day after Momose first sees Yashiro and spots the nailpolish that Yashiro still has on, he "google" searches and reads a "wiki" article on "gender identity disorder." Gender identity disorder is the distress a person experiences when their sex (biological organs) do not match their gender identity (how they see themselves). Someone who experiences "gender identity disorder" can only be transgender. So it isn't so that Yashiro is a guy and Momose is a guy and both guys fall in love (which would be a a "gay" relationship or BL). It's that Yashiro feels "she" is a "girl" that happens to be stuck in a "male" body and Momose falls in love with that "girl" so its actually a heterosexual relationship. I mean, they had been in the same class before but "Momose" never actually falls for Yashiro before he sees "her" cross-dressed (or has a female gender expression). Yashiro also keeps on emphasizing she is not "homosexual" which would imply she sees herself as being a woman (gender wise) and so is not gay (she is a female that is attracted to males). When Momose says he wants to do everything "normally" he means he wants to date "her" and be a in "heterosexual" relationship with her. Thus, from beginning to end, the relationship is a heterosexual one and not a "gay" one just because both leads are "physically" male.
This is a excellent drama that is very good even though its a bit old. You should watch it if you're into mysteries and investigation type dramas set in the historical setting. .