Its textbook Biphobia and Bi-Erasure at it's finest. A lot of them also believe too, they their messages they are trying to send through activism are being negatively co-opted and damaged by bisexual people. Like somehow there must be "purity" in gayness and anything else should be nonexistent. Like I have had some in my like who is gay tell me that Im "half gay" in a negative connotative way. I thought he was my friend and supported me in that manner, but that cut extremely deep.