The official term for moments like these is "bisexual erasure," and it's just what it sounds like. These shows are erasing bisexuality as a valid sexual orientation. We see it in Buffy The Vampire Slayer when Willow — who had a loving relationship with a man for three seasons — starts dating a woman in season 4 and is labeled a lesbian for the rest of the series. It also pops up in the first season of Orange Is The New Black when Piper describes herself as "a former lesbian," in Sex In The City when Carrie Bradshaw says , "I'm not even sure bisexuality exists. I think it's just a layover on the way to Gaytown," and in countless other shows.