A figure in a translucent kimono coyly holds a fan. Another arranges an iris in a vase. Are they men or women?

As a mind-bending exhibition that opened Friday at the Japan Society illustrates, they are what scholars call a third gender — adolescent males seen as the height of beauty in early modern Japan who were sexually available to both men and women. Known as wakashu, they are one of several examples in the show that reveal how elastic the ideas of gender were before Japan adopted Western sexual mores in the late 1800s.

The show, “A Third Gender: Beautiful Youths in Japanese Prints,” arrives at a time of ferment about gender roles in the United States and abroad. Bathroom rights for transgender people have become a cultural flash point. The notion of “gender fluidity” — that it’s not necessary to identify as either male or female, that gender can be expressed as a continuum — is roiling traditional definitions.