One's sexual orientation or preference is one's CIVIL RIGHT, and one's civil rights cannot be discriminated against. You can try to twist it any way you want to, or make something extremely technical, when it is quite simple.

The march and fight for Civil Rights were for ALL PEOPLE, and its enactment is for ALL PEOPLE.

CIVIL RIGHTS. The rights of the CIVILIANS of the united states of America that no one can be discriminated against due to their RELIGION( Beliefs or way of life) or SEX.

And as I stated, one's sexual preference or orientation is a way of life or belief, so therefore it can be perceived as a religion, so it is protected on that element as well.