Homosexualist. "One entry found." Only one? Merriam-Webster is perhaps being a little coy here?
Let's see what usage examples our good old OED can give:
1931 A. EILOART tr. Heyer's Hypnosis xiii. 204
It should be considered that a homosexualist thus suggestively transformed for weeks or even months, marries..within that time.
1933 H. WILLIAMSON Gold Falcon i. 19
His book..proving..that Hamlet should be played as a homosexualist in vain love with Horatio had just been published.
1961 Innocent Moon v. 100
Even Jesus.. according to Nordau..was a homosexualist and should have been locked up.
1971 Southerly XXXI. 84
He consigned me to the lavatory-cleaning brigade together with a group of homosexualists.
What lofty fellows you associate with, Mr. Grew!