Very nice post Colleen and I agree 100%. My best friend of 25 years is gay and he makes many of the same points. However, I still feel like this is an unfair critique of a film. 500 days is not trying to say anything about homosexual relationships. In fact, given it has a narrator, it is explicitly about his own life. He is hetero and therefore for him there are only two types on genders and sexes, male and female. Unfortunately, this is the view of the majority of people. So of course it is heteronormative.
I think that is a FAR worse crime to address LGBT issues when you really have no interest in them like, all Adam Sandlar movies, Bruno, or Kissing Jessica Stein.