There is a gulf between a personal preference and a hard and fast rule when it comes to whom you date/marry. A personal preference is flexible enough to allow you to give a person a chance as a total package. A hard and fast rule means no matter who the person is as a total package you refuse to give them a chance.
I. Personal Preference: I prefer to date cisgender people. (finds out romantic interest is transgender) I like you, so I’ll continue dating you.
Hard and Fast Rule: I prefer to date cisgender people. (finds out romantic interest is transgender) It’s over, no matter how much I may have liked you before realizing you are trans.
The first is a preference. The second is transphobia.
II. Personal Preference: I prefer dating fat people. (feels attracted to thin person) I like you, so I’ll continue dating you.
Hard and Fast Rule: I prefer dating fat people. (feels attracted to thin person) I refuse to go there, because you are not fat.
The first is a preference. The second is fetishizing. Swap “fat” and “thin” and it becomes fatphobia.
Respect your and others’ preferences. Question and examine your and others’ hard and fast rules.