I guess the person is implying that for something to be of the Yuri genre, it would have to be something that's geared to an audience looking for girl-on-girl related-stuff, meaning the fact that Yuri-stuff is incidental to Murcielago, it shouldn't be considered "Yuri."
I don't really agree with this though, but I guess we all define things differently.
I think that's what the first responder was implying, but not the second. I think the second was referring to the frequent criticism of a particular way of writing female action characters as basically "men with boobs." That is, the author gives them all of the behaviors, vices, strengths, weaknesses, and so on of a male hero, but the form of a sexy woman. Oftentimes she's the token "girl who's as good as the boys" on an otherwise all-male team.
In this case I kinda think the critcism fits - Kuroko lusts after and approaches women sexually in a way that is utterly unrealistic for women to do, and of course has an affinity for causing pain and violence that is also very atypical for women. She behaves in almost every way like a male character in her role would. She's also physically imposing and, of course, by virtue of her pathology, devoid of any of the emotional traits that are often associated with female characters.
But of course, under no circumstances can you pretend this story is striving for anything like realism. Did the Kuroko character have to be a woman? No. Would anything really have changed if she were a man? Other than the Virginal Rose arc, no. Does it matter? Not especially. The artist wanted to draw lesbian sex, and that's just fine with me. :)
Hummm I don't agree (your opinion is respectful, I just share the mine). I think the manga will be totally different in that case. I love murcielago and Kuroko because she is crazy, she loves murder, she is in apparence a woman like some characters, fine but for moments... She said things that I can't imagine in a men, because... I don't know how to say it but in the manga appear crazy men and they haven't something dark but not with murders or something like that... Like, with her life's vision. I don't think all she do and her life's vision could fit fine in a men because that madness fits better in a woman (on other hand, it's clearly she could go into men but I preffer the manga and Kuroko like it is now :P yuri power!)