Race is the way you are born - you can't change that. Being gay on the other hand in my honest opinion is down strictly to personal choice. Being racist implies you attack the other person based on whom they are a person, on the other hand if someone is gay, you as a homophobic may attack them upon the basis of the choices they make - being gay.
Now being attacked for whom you are as a person and that you cannot change is different for being criticised for a choice you make - making a choice means it separate you from the object you speak of, in comparison to being of a race, which is inseparable.
More people are affected by racism in the world, and it has far stretching consequences. Homophobia, in comparison, the consequences to the society are less detrimental.