Is Yuuki bi/homosexual?
As someone who's only ever watched the anime series, and someone who's actually consulted some outside help to better explain this, I can reasonably answer:
No.
It isn't uncommon for two girls or two women to hold some kind of affection or level of respect for one another in some capacity.  From my sources, to suggest that they would consider &quot;making babies&quot; with each other implies an opinion that this woman is indeed attractive.
However - and this is a point my source thoroughly stressed - this does not, in any way, imply any form of sexual orientation.
Further to this, given the interactions that Yuuki had with Asuna throughout Mother's Rosario, my impression is that Yuuki held admiration, respect, and love - the kind of love that you'd get from a close family member.  The rivalry between her and Kirito can be seen as more friendly than her trying to one-up Kirito in an effort to win Asuna over.
I also sense a level of admiration and respect from Yuuki of Asuna, given that

 Yuuki is terminally ill and can't really explore the world, which prompted Kazuto to develop technical equipment to allow her to see the world in which she previously existed in, with Asuna's help.  This helped her gain more closure with her situation and, in my assessment, feels more like something an older sister would take on.

With that, I can safely and reasonably conclude that there's no implication whatsoever of Yuuki's sexual orientation.  Respect or admiration of someone who seems awesome from their perspective seems more plausible.
Oh, and don't read too deeply into what fanart depicts.  Give 'em an inch and they'll run to Jupiter with one or two fleeting glances.