I dont get all the excitement about Will's sexual orientation. Maybe it was a big deal back then for the viewers to see a gay relationship portrayed, but for Buffy, Xander and Giles Tara was as good and acceptable as Oz and Will being gay didn’t make any difference. I guess in high school it'd be the same.
As about Will breaking up Xordy, i think if not that kiss in LW, then something else would do it. It wasn't the strongest relationship. Xander wasnt that invested in it. We'd miss some Will pining after Xander, but that's all.
Besides all those discussions in various threads whether Willow really loved Oz if she was really gay from the start seem pretty pointless to me. Things change over time, and besides i think she loved the person (Xander, Oz, Tara, Kennedy) regardless of their gender.
My personal theory is that Will is pansexual, and she defines her orientation in regard of her current partner. In Tabula Rasa for example, when she is in love, she looks at Tara, recognizes her attraction and concludes 'i think i am kinda gay'.
Also in the infamous case with RJ's jacket Will isn't troubled the least him being a guy until she is specifically told by her friends that she cannot love RJ, him having the wrong gender and then suggested by Anya that she should turn him into a girl. If not told she'd be probably perfectly fine to pursue a guy.
but at this point i think the fans would strongly object of Will having anything but a lesbian relationship, so that's idle talk.