Idea
The Pun Tyrant
If it was contained just to a self-aware contained version like the character merely being convinced their identity is built on their sexuality and acting accordingly, I would be inclined to agree with you. But what is often the case is that every single aspect of the character is warped for the message, and again, this wouldn't be entirely unrealistic, except that one of the things most commonly changed is the backstory. It's not just the character themselves which are warped for the message, but the behavior of the entire plant seems to be designed to glorify whatever the message might be and villify opposition. For instance a "messenger character" on the topic of abortion might have every single bad thing that happens to them be directly caused by the abortion or by people who typically are against abortion such as priests. One for a disorder of some kind will be extremely specific on what that disorder is on the character (typically because the player themselves has it, but regardless), almost every single entry will reference the fact they have that disorder and the backstory will center around the fact the disorder is there, quite often the person being discriminated against or just a particular problem of the disorder, which again will appear to shape every bad thing in the character's life.To be fair there's people like this in real life. There's people who actually do seem to build their entire identity around their sexuality or some cause they care strongly about. Sure, it might make for a boring RP character, but it's not inherently unrealistic. That being said, preaching is better left for one's personal writing, imo.
It is understandable that someone might overly rely on one aspect of their identity. I might even praise people who make such a character, as doing so in self-awareness would probably indicate they wanted to explore the matter, both the good and the bad. A "messenger character" takes this to the extreme where every part of the character and everything around them gets warped in this bubble of agenda.
I don't know if this will help any, but what I typically use the likes and dislikes for isn't for a comprehensive list of everything a character likes or doesn't like, but rather as a sample of likes and dislikes meant to help organically bring to the table certain reactions, as well as flesh out/humanize the character to an extent. For instance, using a like you can generally expect a positive reaction from the character, so it could be used to help break the ice between two characters. A dislike might serve as an organic way to get a character angry/upset or to have them not interfere in a plot point which might've otherwise been resolved too easily. Etc...I also don't like "likes" and "dislikes" lists. There are many things that a character is going to like and dislike.