Can "homosexual" be used as a noun as in these sentences?

1. The ADA prohibits discrimination against a homosexual who is HIV-positive and whose job application was rejected because he or she is HIV-positive.

2. A subsequent War Department medical bulletin warned examining psychiatrists to be on guard against the homosexual who may see in the WAC an opportunity to indulge her sexual perversity.

3. Mexican society remains highly prejudiced against homosexuals who are HIV-positive.

Are "a homosexual", "the homosexual" and "homosexuals" all correct?