They're not?
Gender discrimination: "She's too emotional, and she'd call in sick once a month. We can't hire her."
Race discrimination: "I didn't rent the apartment to him because he'd be playing that loud 'music' all the time."
Age discrimination: "Let's not hire him. He'd be too slow."
Disability discrimination: "Don't bother taking her order. I'm sure she can't talk clearly anyway."
All these forms of discrimination are based on assumptions about the behaviour of an individual based on their membership in a particular group, and come down to the fact that the person doing the discriminating is uncomfortable with that group.
Ok, I get it now ... it is just a stereotypical assumption that homosexuals want to have same sex relationships
Homosexuals are defined by behaviour (real or desired) in a way that other minority groups are not. A blind person is not blind because he wants to have a guide dog!
Behaviour is either moral or immoral. Therefore it is my responsibilty to decide what I believe is right and wrong, and act accordingly.