Biphobia in Gay People

Before realizing that I was gay, I identified as Bisexual to those I'd come out to. This wasn't to "soften the blow" or "test the waters" but as I actually thought that I was Bi. Anyway I have since then come to the realization that I am gay, and I am catching myself having thoughts that are obviously not okay. Like, when I identified as Bi I didn't understand Biphobia in gay people, and to a certain extent I still don't, but NOW whenever I hear that someone is Bisexual in my mind I'm all like "yea I thought that too once ahahaaa" and then I'm like "wait wtf that was biphobic" and also like "woa dude dont think like that just coz ur gay doesn't mean other people aren't bi". But those thoughts do enter my mind pretty naturally and I'm wondering what it is in some of us that are inherently biphobic after coming out as gay. What do you guys think?