Gender has not always been understood or experienced as fluid — in fact, for much of recorded history, many societies operated under rigid gender binaries that defined roles, identities, and social status in strict male–female terms
So, says you or whomever you are quoting. But you are not entirely correct. While the term gender fluid is a fairly new one having started around 1990. And Victorian society opposed homosexuality and gender fluidity while rigid gender roles defined daily life. Victorians were prudes. But than, Victorians were sexual hypocrites because the sexuality they condemned publicly many practiced privately. The practice and acceptance of gender fluidity dates back to ancient civilizations.
Mesopotamian texts from thousands of years ago describe worshippers of the goddess Inanna who moved fluidly between male and female social and spiritual roles. Ancient Greek mythology embodied both male and female sexes or characteristics, most notably Hermaphroditus and a distinct aspect of the goddess Aphrodite. Numerous societies historically incorporated roles outside the strict binary. Examples include the Hijra in South Asia, Fa'afafine in Samoa, and Two-Spirit identities across various Native American tribes. Some traditional African and Mesoamerican societies included spiritual practitioners or diviners who dressed as and lived as the opposite gender or combined aspects of both.