But Willow's not a real person. Willow's a fictional character, created by people who are the product of their times. In real life, it would be intrusive to speculate at length about whether Buffy really loved Angel, or Riley, or Spike. Speculating about real people can be a way to gather information about how people work - as fiction can only imitate life so much - but it's not fair to the people involved. In fiction, it's acceptable. Fictional characters are not real people.
In fact, it's more than acceptable to question how fictional characters self-identify. Fiction is a mirror of life, and trying to erase the parts of life that make us uncomfortable is not going to help anyone. It's just going to make the wrong people feel ashamed.
The reality is, it's not always a good idea to take a person's word at face value about their sexuality. Don't get me wrong, trying to tell a friend, family member, or total stranger what their sexuality "really" is would be arrogant and intrusive. But sometimes people don't feel safe outing themselves. It's not necessarily fair to put the onus on the closeted person to decide if you are a safe person to approach. Many of these people have been badly burned. Asking in an empathetic and respectful manner is not intrusive. Just remember that the person being asked gets to decide what is empathetic and respectful, not the supposedly well-intentioned asker.
Getting back to Willow: Prior to Tara's introduction, we see almost no indication that Willow is attracted to women. Willow has her long-term crush on Xander, which includes a period of time where they are unable to resist kissing, and then an eventually sexual relationship with Oz that lasts from the middle of S2 into S4. If I recall, we never see any indication that Willow is dissatisfied with the sex. I also recall mention of a crush on Giles. The only indication of attraction to women that I can recall is Doppelgangland - and that's AU Willow, not main world Willow.
Sure, it's possible that Willow was a lesbian all along, attracted exclusively to women, and all the hints are off screen. But if we take the evidence of what we see, Willow is attracted to men.
Does that mean I think Willow is bisexual? People throw around a lot of labels about sexuality. Bisexual, pansexual, omnisexual. We say sexuality is "fluid", and yet the intense debate over the right word for Willow continues. Yet, even most people who consider themselves 100% straight or gay will say they aren't attracted to everyone of their gender, and they will admit an attraction to the occasional opposite-gendered person. I'm thinking these days that the focus on gender is a distraction - some people have a gender preference, but it's not the only factor in sexual attraction. And perhaps it's not the most significant one for most people.