The perception that "unisex" fashion arrived in the 60s then went away and came back.

Here's "A Brief History of Unisex Fashion" in The Atlantic:

In her new book Sex and Unisex: Fashion, Feminism, and the Sexual Revolution, the University of Maryland professor Jo Paoletti revisits the unisex trend....

As far as the American fashion industry was concerned, the unisex movement came and largely went in one year: 1968. The trend began on the Paris runways, where designers like Pierre Cardin, Andre Courreges, and Paco Rabanne conjured up an egalitarian �Space Age� of sleek, simple silhouettes, graphic patterns, and new, synthetic fabrics with no historical gender associations....

The unisex movement may have made women�s clothes more masculine, but it never made them unfeminine; furthermore, �attempts to feminize men�s appearance turned out to be particularly short-lived,� Paoletti notes....

Paoletti traces the end of the unisex era to the mid-1970s. In 1974, Diane von Furstenberg introduced her wrap dress, a garment that combined femininity and functionality....

Since the 1990s, however, fashion has been blurring gender lines once again. A recent New York Magazine story traced modern androgyny to grunge: Women donned flannel lumberjack shirts and combat boots while Kurt Cobain posed in ballgowns and housedresses.... Indeed, unisex everything appears to be back with a vengeance....

As fashion, unisex must cycle in and out. Things must look new and then old and, eventually, new again. But not all unisex clothing is part of fashion trending. Once everyone decided we could wear jeans (and other workmanlike trousers) and T-shirts (long and short sleeved), that was always an option, not because of fashion, but for functionality, cheapness, and simplicity. Fashion sometimes taps into this anti-fashion work-and-poverty ethic, so it seems to go in and out, but it's really always been there. It's a separate question whether anyone is specifically attempting to project the message: I am neither male nor female. A subquestion is: Among those who choose to project a neither-male-nor-female message, which ones are expressing his/her true identity and which ones are trying to be cool/cute? Subsubquestion: Which ones even know?