Green’s Dictionary of Slang

(US gay) effeminate, homosexual.

1965		Guild Dict. Homosexual Terms 29: mary (n., adj.): [...] In its adj. form, ‘Is she ever mary,’ it states that the male homosexual is very feminine, or ‘Mary, you’re a camp.’.
1972		B. Rodgers Queens’ Vernacular 131: Mary [...] 2. (adj) feminine acting.