Maybe an avatar looks female, but actually the character is a pre op FTM who prefers "he". Or maybe the character is neither a man nor a woman and prefers "xe".

The OP asks people to assume that gender matches appearance stereotypes, but that's hurtful to characters with other types of gender.

It is unrealistic to expect strangers to know how who you are and what your preferences are. They don't know your IRL gender and they can't be sure of your character's gender based on appearance.

Don't be hurt because strangers don't know you. Unless you ask everyone you meet what gender they are and what pronouns they prefer, then admit that it's a bit unrealistic to expect people to go that far.
People make assumptions so they can communicate and play the game, not because they're trying to hurt you.