I've had this idea ruminating in my head for a while. The debate, I'll admit, seems a bit flimsy and I'm not sure how well I'm going to be at defending it. But here goes...
The term sexism is defined as prejudice or discrimination based on sex. US Citizens are protected against that type of discrimination by Title VII of the Civil Rights Act of 1964
which includes sex amongst race, color, religion, and national origin. The act begins:
To enforce the constitutional right to vote, to confer jurisdiction upon the district courts of the United States to provide injunctive relief against discrimination in public accommodations, to authorize the attorney General to institute suits to protect constitutional rights in public facilities and public education, to extend the Commission on Civil Rights, to prevent discrimination in federally assisted programs, to establish a Commission on Equal Employment Opportunity, and for other purposes.
Homosexuality is defined as: of, relating to, or characterized by a tendency to direct sexual desire toward another of the same sex. (Heterosexuality having the same definition except for "opposite sex" replacing "same sex".) The only difference between these two distinctions is the sex of the other person. Since the sex of the other person is the defining part of what makes the difference, how could this not be sexism? And since it's against the Civil Rights Act of 1964 to discriminate on the basis of sex, shouldn't that make it de facto illegal to discriminate against homosexuals?
I do see the problems inherent in the proposal however. The discrimination is only in correlation with the secondary person and as such, is not direct discrimination.