Why does this matter anyway? Isn't it just entertainment?
Yes, but entertainment is our common cultural currency. It's where we see ourselves reflected, and it's one of the primary ways we learn about people who are different from us.
When we are reduced to our sexuality, we are seen as other — something less than full human beings, and therefore less deserving of equal rights. When we are not presented three-dimensionally, it becomes more difficult for many heterosexuals to identify with us, because there's no common ground.
Whether it's making it more difficult to find LGBT books on Amazon because they are classified as "adult," preventing a high-school gay-straight alliance from meeting because the school doesn't sanction sexual activity, or making it easier for anti-gay activists to block gay marriage, these media images have a negative impact on our lives.