There shouldn't be a need for reason or justification for a queer (lgbt+) character to be included, because they exist, just like that, among us. I find people's excuses for not including one quite laughable, especially when this manga has the perfect setting for such inclusion. I mean, come on, Fushi is an immortal, shape shifting, sentient being who has no gender or sexuality. That doesn't mean they can't have either of those things, it means they can have and be anything. A blank canvas.
As for Todo, I do think it was quite odd to make her a woman, after all that. I don't have any "desire" or "motive" to make them a gay character, but having one would be a nice nod to recognize and acknowledge their existence, even in a fictional/fantasy setting. Also, Bon is quite a queer character himself, eccentric yet tolerant of all. He has no qualms with homosexuality or loving an inanimate doll, and he is a lovable, respectable person. Having that acknowledgement alone is important.