Oz had nothing to do with it, nor do I believe that it was actually any sexual orientation affirmation. Willow summed it up perfectly when she said, "It wasn't women. It was one woman. Tara." And that's all that I ever saw when I watched them, and all that I ever thought. Willow and Tara were two people who fell in love and who lived a very important part of their lives together. It wasn't about gender or sexual orientation, it was about two lonely people meeting and falling in love. That's why Willow and Kennedy never worked for me. Willow was lost and confused, very vulnerable and lonely. Kennedy did everything short of tearing her clothes off and... Well, let's just say that she was far, far away from anything resembling subtlety in making it quite clear that she was attracted to Willow, and, by her own admission, she always got her way. (I've often wondered what would have happened if "her way" had been Buffy or Dawn instead of Willow) Personally, I always assumed that at some point Willow just said to herself, "What the hell, we're all probably gonna be food for the worms in a matter of weeks anyhow. Might as well have a little fun in the meantime," and that's all that she ever wanted from the relationship...
... and it just occurred to me that I have drifted off of the subject again.
Anyhow. As I understand it, no one have their true sexual orientation changed forcibly by another. Straight, gay, or bisexual,you're pretty much born with it, you live with it, and you die with it. So Willow's experience with Oz didn't make her swear off of men.
It did, however, almost certainly make her swear off of werewolves.