Como dizer "Pessoa relativa" em inglês

"Pessoa relativa" não tem nada a ver com "não binário".
O termo usado em inglês para pessoa relativa seria "Versatile".

"Vers (short for versatile) is a term used for men who don’t have a position preference when they have sex with other men."

Ref. metro.co

There are many ways they call themselves, and many come up by the minute.
However, it's likely: NB or enby (no binary) and its corollaries (subsets) - genderfluid, genderqueer, bigender, gender-neutral.

Today, there's a small distinction between gender and sex, sex being the natal sex (sex of a baby at birth) male, female or intersex (a person that may have genitals or internal sex organs that fall outside of typical binary categories).
Gender having, to do with social identity and expression, sex with Biology (the science subject).

I see. Thanks for chiming in, Andrezza.

Versatile - switch