While Sarah Michelle Gellar led Buffy the Vampire Slayer as the titular slayer, Alyson Hannigan's Willow was one of Buffy's best friends and had a major role in all seven seasons. Willow's sexuality was a major arc in later seasons, however, Whedon said he'd have made her bisexual from the very beginning if he could do it all over again.
Speaking to Metro, Whedon explained he was pressured into avoiding a bisexual Willow, over fears it would be classed as a "phase." "There are you [sic] things you can't do, thanks to [the society at the time]," Whedon explained. He continued, "It [was] like, 'OK, you can't make Willow bi, you can't say this is a phase, because that's what people do to deny their existence.'" Discussing what he would do differently, Whedon concluded, "So, if I did it now, I'd be like yes she can be bi. Because some people are! But back then it was like, 'No…we're not ready for that.'"
Instead, Whedon made the choice to have Willow come out as gay. It was a decision that has led to some confusion in the TV series and the expanded canon of Dark Horse's comics. Willow's sexuality became as much a part of the character as her magic and resulted in Tara's tragic death/Season 6's Dark Willow storyline.
Back in Season 4 of Buffy the Vampire Slayer, the introduction of Amber Benson's Tara facilitated Willow's change in sexuality on the show. Prior to dating Tara, Willow had heterosexual romances with other characters, including Xander Harris and Seth Green's Oz. Buffy the Vampire Slayer was a pioneering show with Tara and Willow's relationship-- even depicting broadcast TV history's first lesbian sex scene. Still, Whedon's comments about the industry not being ready are indicative of just how much the industry has changed over the years.
A modern reboot of Buffy the Vampire Slayer has been in the works since 2018, with Monica Owusu-Breen attached as showrunner. Rumors suggest that the series will be a continuation that will focus on a new slayer, which means there may not be a new Willow. Hannigan has previously spoken about the reboot and said it could work, but only with Whedon's involvement. She went on to applaud the original series for being ahead of time and still thinks it holds up today.