Another option is camp (adjective), meaning to behave in a way stereotypical of a gay man (in Western culture). This includes exaggerated feminine traits, but also some traits (mannerisms, walk, accent) unique to the gay male subculture.
Note that gay itself could be an option, if you genuinely think they are homosexual. However this has also been used as a general insult/criticism without necessarily being related to sexuality. "That's so gay" implies that you think something, or something someone is doing, is bad/stupid/lame/dumb. Katy Perry wrote a song about an ex-boyfriend called "Ur so gay (and you don't even like boys)" which made the intent of the insult clear. This use is generally offensive to gay people, of course, because it originates in accusations of homosexuality being used as insults, and then became a more generic insult in the 1990s when homosexuality became less taboo.