Start by assuming the best in others.
This is a helpful attitude to maintain when you want to make peace out of conflict. The first step in resolving this conflict should not be “How do I convince them they are wrong?”. Rather, we should first try to see if we are misunderstanding them when they are actually trying to help.
Allow me to explain a possible, positive motivation behind this ruling.
Sexual and gender identity transcend what the game calls “Personality Traits”.
Up front, I must affirm: make the character you want to make. As a GM, my goal is to facilitate fun at the table, and I’m not going to tell you who your character is. That isn’t my job. It is the GM’s job to tell you who the NPCs are. Characters uniquely belong to their players.
That said, gender and sexuality are things that are fundamental to the identity of the character. They are a large part of who the character is. These things influence how your character relates to and interacts with the rest of the game world, all of the time.
But these things will not necessarily make your character unique.
If I am the DM helping you work through character creation, by all means, make the character you want. But I would suggest that you are losing out on a way to set your character apart by writing your gender identity and sexual orientation in the Personality Traits section of your character sheet. These things are as fundamental to your character as your character’s name, yet you wouldn’t write, “My name is Bardic Wizard” in the personality traits box. The Personality and Background section of the Player’s Handbook says this about personality traits:
Personality traits are small, simple ways to help you set your character apart from every other character. Your personality traits should tell you something interesting and fun about your character. They should be self-descriptions that are specific about what makes your character stand out. “I’m smart” is not a good trait, because it describes a lot of characters. “I’ve read every book in Candlekeep” tells you something specific about your character’s interests and disposition.
Personality traits might describe the things your character likes, his or her past accomplishments, things your character dislikes or fears, your character’s self-attitude or mannerisms, or the influence of his or her ability scores.
The things described here are not fundamental to the identity of your character. They make your character unique in relation to other characters, but these things are not your characters fundamental identity. As a DM, I would encourage you not to reduce your character’s identity to what is described here in the Personality Traits section. Sexual and gender identity transcend this box. Use this box for something else.
This is probably intentional. Sex and gender are addressed in a separate section of the character development chapter. In particular, the Player’s Handbook has this to say:
Think about how your character does or does not conform to the broader culture’s expectations of sex, gender, and sexual behavior. For example, a male drow cleric defies the traditional gender divisions of drow society, which could be a reason for your character to leave that society and come to the surface.
You don’t need to be confined to binary notions of sex and gender. The elf god Corellon Larethian is often seen as androgynous, for example, and some elves in the multiverse are made in Corellon’s image. You could also play a female character who presents herself as a man, a man who feels trapped in a female body, or a bearded female dwarf who hates being mistaken for a male. Likewise, your character’s sexual orientation is for you to decide.
So in a strictly rules as written sense, you can be whoever you want.
Even if this is not Lemon’s motivation, I hope this can give you a positive outlook on how to digest the situation.
Have a conversation with Lemon: character sheets are not characters.
Maybe Lemon’s motivation is less positive than what I described above. Either way, we need to have a conversation with Lemon.
Lemon needs to understand something fundamentally important. Character sheets are not characters. Character sheets are a tool used by player’s to help them interact with the game. The character is the person that you bring to life at the table.
It doesn’t matter what your write on your character sheet. The Personality Traits section of one of my characters only has two words: fire and sarcasm. These are just little things that set my character apart and hep me inform my roleplaying in little ways every now and then. This is how the character sheet should be viewed. It’s just a tool for the player to use to relate to their character and the world.
You may have to just show Lemon what maturity looks like.
If Lemon refuses to budge, and insists on not being allowed to put your sexual or gender identity in that box, just move on. It’s just ink on paper. At this point, Lemon has not told you your character cannot be who you want them to be. They’re just trying to micromanage words on a page. Is it stupid? You betcha. Is it worth getting upset over when you can otherwise create the character you want to create? Probably not. Maturity will see a peaceful compromise in picking out some other personality traits and being the person you want to be at the table.
And if this is the route you take, don’t announce it to Lemon. You don’t have to say, “I’m going to be the mature one and move on.” Your graciousness and humility will say everything that needs to be said, and hopefully, fun will be had by everyone.