I have had this conversation too many times, and it is very frustrating. You are absolutely right to feel gas-lit.
Of course having a personal policy not to date bisexual people you would otherwise be attracted to is biphobia. It is the very definition of biphobia.
Just like it is the very definition of racism for a white person not to date a black person they are attracted to.
Tell your friend that the critical distinction here is the element of attraction. Of course, no one can force themselves to be attracted to someone they aren’t attracted to.
If he chooses not to date a bisexual person because he isn’t attracted to that person, that is obviously not biphobia.
But if he is attracted to a person and makes a choice not to date that person because he doesn’t like the idea of dating a bisexual person, then he is biphobic by definition.
Is that a preference? Of course it is. Homophobia, transphobia, racism, and biphobia are all preferences. We all have to choose to be those things.