In a newsmagazine cover article on Gore Vidal in the late 1970s, the celebrated author and essayist explained that, since "homosexual" is used as an adjective ("homosexual fantasy"), the noun form needed something more, well, distinctive and substantive: he used "homosexualist" to describe someone who is gay in practice, or as a state of being.
One doesn't argue lightly with Gore Vidal but there are precedents either way in forming nouns. "Alcoholic drink" / "Joe's an alcoholic," uses "alcoholic" first as an adjective, then as a noun. Similarly, "Green politics" / "Cary has become a Green."
OTOH a medical practitioner of psychiatry is not a "psychiatric" (better used as an adjective = "psychiatric evaluation"), but a "psychiatrist," a description of a person, not a field. One who enjoys sensual things is a "sensualist" but has an appreciation of the sensual.
"John is an out-of-the-closet homosexualist."
BUT ALSO: "John is a homosexual," OR
"John is homosexual." -- BUT NOT:
"John is homosexualist."
It is much to be hoped that the definitions above of "same-sex love" or "practitioner of same-sex love" will stand, despite the fact that many right-wingers use it almost as a slur (it can get clinical) and avoid "gay" as a neologism. Don't think it isn't political, either.
GAY: Current idiom in casual speech would have it
"John is gay." Note that "a gay" commodifies John just a little.
"John's love life has been exclusively homosexual since 1993."
"John has an active homosexual love life."
"John is the kind of homosexualist other homosexualists can be proud of."