Captain Verd
Well, considering it's a boys manga, I'm gonna go ahead and assume Zabuza was straight. And since he's straight, I'm gonna go ahead and say it would be easier to fall in love with something with boobs than something without.
No offence, but that's an incredibly weak argument. "It hasn't been done before, therefor it will never be done and this can't possibly be it" it too much of a broad statment, and depends too much on probability and not enough on what's actually in the manga. I mean, if you want to use a statistical argument, you might as well insist that I'm Chinese, since I'm human and a majority of humans are Chinese. Of course, I'm
not Chinese, so that just goes to show how effective that sort of argument is.
Kishimoto has already shown that he's not afraid to the teh ghey in his manga. The Naruto/Sasuke kiss is proof enough of that. It was an accident, true, but it still happened, which means he's not afraid to put homosexual content in his work.
Haku has dedicated his life to Zabuza, and dies for him. Haku is the only thing Zabuza really values. Zabuza performs a suiced attack to avenge him, in fact. And when he dies, he strokes Haku's face and wishes to be with him in the afterlife (and gives him the shiney, pretty eyes they always use for Big Emotional Moments). Haku calls Zabuza his precious person, and even tells Zabuza he has a beautiful body. Really, what more do you want? Does Kishimoto really have make an annoucement "yes, these two were gay?" And if so, does he have to make one to tell you the grass is green in his world for you to believe that, too?
And if you really want to use litterary conventions and such for this argument, maybe you want to look up
shudo, which describes their situation perfectly: an older warrior nuturing a very pretty youth. . . and boffing him on the side. It's an old Japanese custom. After all, this is a Japanese manga, written by a Japanese man, who has not been steeped in Western prejudices.