That assumption isn't valid.
In some languages in order to say anything about anything you have to apply either the masculine or feminine form of the verbs and adjectives, so there's no way around picking one of the two, so if you don't know, you must guess or assume.
And if we look at languages with three grammatical genders, for some of them the neutral option is exclusive to inanimate objects, and using these neutral-gender forms of verbs and adjectives involving any person is explicitly insultingly dehumanizing, you wouldn't even use it to refer to an animal, much less to any person unless you want to make a point of intentionally disrespecting them as "it", saying that they are not worthy to be considered a person. In these languages[1] we do see the neutral gender option applied towards nonbinary and transgender people, but it's done by transphobes as a way of dehumanizing them in conversations.
In such languages usually the proper way to behave if you don't know the gender or if that's a mixed gender group is to default to masculine grammatical gender; because there's no polite way to avoid choosing; and, I might guess that there's also probably some aspect that historical patriarchal social reasons it might have been traditionally considered that misgendering someone male-to-female is more offensive than misgendering someone female-to-male, so that's how that default might have come to be.
[1] edit - I know about some of these languages, I'm not asserting that this applies for all such languages, perhaps there are some of them where the social norms are different, it's very hard to generalize globally.