as a bi dude, i think this is a younger/recently out bisexual thing to do. i used to say it because i just couldn’t really fathom someone not being into both even if it was just a teeny tiny bit due to my own experience with sexuality. i dated a man briefly who would physically recoil when i talked about my experiences with women and that’s when i realized that yeah, some people really just are 100% gay or 100% straight, even if it doesn’t compute in my own brain lol.
Like I used to identify as bi before I came out as a lesbian, bc I tried to force myself to like men. Since I have come out as lesbian: when I have sex I no longer feel like self harming after (something I felt every time I was with a man), I can get turned on from kissing alone (this never happened when I was with men), I don’t feel empty going on dates and I don’t feel like I’m putting on a show anymore. And yet I still have people trying to tell me I’m bisexual a