While I agree that having a sexual preference constitutes a "bias", to call it a form of "homophobia" or "transphobia" (however benign) renders those words meaningless (and is simply inaccurate), as mere sexual preference (or "bias") in and of itself is not an expression of hatred or fear, and hence not a -phobia.

Ultimately, this insistence that there's something wrong (even if "more or less benign") with deviating from a position of pansexuality implies a sense of entitlement to other people's bodies, or to dictate who they're allowed to find attractive. While you say you don't consider it necessarily a bad thing for a person to have a sexual preference, you're nevertheless attaching terms to it (i.e. homophobia and transphobia) that carry a lot of baggage for most people, and are implying that there is some sort of moral transgression behind a person simply following their own natural impulses in who they feel (or don't feel) attracted to.