Generally speaking, you're both right. A reader should read mindfully. Great fiction relies on revealing information through action, dialogue, word choice, description, and more. Often details suggest deeper meaning. Putting it all on the surface would render a story simplistic and boring. Still, the author has a responsibility to offer up details that are precise and don't distract or mislead. Even a single word choice—laden with strong connotation that you don't consider—can send a reader on a very different path than the one you intend. So, before you take your fellow writers to task, figure out if they're actually making assumptions or if you've done something to lead them to the conclusion they made.
Start with the opening paragraphs. It's important to establish the basic attributes of the narrator early. If you wait too long, the reader may make a conclusion that's too set to dislodge later. Some authors approach this quite directly, as ZZ Packer does in the first line of her short story “Brownies:"
By our second day at Camp Crescendo, the girls in my Brownie troop had decided to kick the asses of each and every girl in Brownie Troop 909.
The narrator in Jhumpa Lahiri's short story “The Third and Final Continent" reveals this information while describing his living quarters in London: “a house occupied entirely by penniless Bengali bachelors like myself."
Frank O'Connor begins his short story “First Confessions" with a description of the narrator's grandmother, a woman who was a “real old country woman and quite unsuited to the life in the town." The second paragraph begins with this revealing line:
Now, girls are supposed to be fastidious, but I was the one who suffered most from this.
Myla Goldberg's “Going for the Orange Julius" begins like this:
It's not only about looking good. If you're just looking good, you'll probably be able to get a cone or a soft pretzel, but definitely not an Orange Julius.
Maybe you're beginning to think this is a woman—many readers do. The next line confirms this:
“Carrie," Grandma says to me as we walk into the mall, “are you feeling like a lady?"
Edwidge Danticat uses a strong feminine voice in her short story “Night Women" to establish the narrator's gender:
I cringe from the heat of the night on my face. I feel as bare as open flesh. Tonight I am much older than the twenty-five years that I have lived. The night is the time I dread most in my life. Yet if I am to live, I must depend on it.
Certainly, we're taping into cultural norms and understandings in drawing some of these conclusions. (Brownie Troops are made up of girls. Very feminine voices often belong to women.) As an author, you need to be aware of this commonly held knowledge. While many options are possible, the reader will be tuning in to what's probable. If your character acts in opposition to common understandings, you'll need to address that quickly.
Of course, the beginning is just that—the beginning. While you should establish basic attributes such as the narrator's gender early in the story, subsequent details should flesh out the characterization. Many misunderstandings arise because subtle choices throughout lead readers astray. As always, be vigilant in avoiding stereotype. Not all women are frilly. Not all men are macho. Resorting to these kinds of details can be just as damaging to authenticity.