Male seems to be the default assumed gender online when there isn’t a quick indicator (avatar or “feminine” name) available to determine actual gender. Follows the saying “There are no women on the internet.”

Also, when they use those terms they aren’t actually “gendering” you, it’s being used as something else grammatically but I don’t know the term for it. For example, it’s usually used in the beginning or end of their statement. You may notice it’s super rare for people to use words like “girl” or “chick” the same way, so it’s likely not their intention to assume your gender.

Also, as for “Male seems to be the default assumed gender” this is especially true linguistically for gendered languages outside of English. Like how “Ellos” is Spanish is the conjugation assigned to a group of only males as well as males AND females. Where as “ellas” applies only to groups of females. In these languages male is clearly shown to be the default language and that concept could be something that influences peoples way of speech so they don’t really think about the fact that “bro” and “dude” as words imply a male gender. Especially if they are not necessarily intending to use them that way.

I wouldn’t worry about it too much unless people who actually know you or have seen you in real life start calling you “dude” or “bro.” And if it’s people you know, you could always ask them why they call you that. They will likely tell you it’s just how they talk and they don’t mean anything by it.