If every movie was made to address LGBT issues and relationships that would be terrible. Why can't you just accept the movie for what it offers?
While I agree with what you say, I also agree with Faceboy's critique. Heteronormative doesn't just mean that it's about heterosexual relationships, it's because themes in the movie suggest that it only applies to hetero relationships, or hetero relationships are the only ones worth considering. It means that it reinforces the overall societal assumption is that the default is for everyone to have heterosexual relationships (normal) and other relationships, whether the take on them is positive, negative, neutral or just ignores them all together, are not the norm = abnormal.
If I understand correctly, it's not so much that it's a movie about a heterosexual relationship as that it makes big sweeping statements about relationships in general without qualifying them as only applying to one particular type of relationship. I haven't seen the movie so I don't know whether I agree or disagree but that's how I read the critique.
And actually it can affect my interest in a movie. When you are part of what is considered the default mode in our society (white, male, able-bodied, etc. etc.) it's hard to notice all the stuff that reinforces that as the default mode. A movie with a female main character is classified as a "chick flick" 90% of the time while a huge majority of "family films" have male protagonists. Little girls are expected to go see a movie like, say, Holes or Hercules and identify with the main character and learn something from what he goes through; there is no equivalent expectation for little boys to watch something like Cinderella or even Mulan and have the same experience.
When you aren't part of the default mode, those assumptions can occasionally rub you the wrong way. Example: family weddings always bring me down somewhat because as much as I'm happy and feeling celebratory toward my sister or brother, there's always an underlying message that this sort of unqualified family celebration with all its traditions will never be for me, even if Jan and I do get married or have a commitment ceremony, and even if some family members come. It will still be weird and uncomfortable and like the people who attend or don't attend will be taking a stance on a political issue as much as enjoying a family party. After a weekend with a family wedding and all of the celebration of heterosexual unions, I'm not going to be very interested in watching a romantic comedy about a male and a female falling in love and getting married. Instead of entertaining, it pokes at the sore spots.
Movies with an all white cast making sweeping generalizations about people in general, expecting everyone to share in that experience, are the same way. And while they may be really good movies and entertaining in their own right, a person of color may overall enjoy the movie while feeling a bit of irritation that, say, John Hughes is speaking for all teenagers instead of only white middle/working class teenagers. How would Pretty in Pink be different if Ducky was black? How would Breakfast Club be different if Anthony Micheal Hall's character was Chinese-American or Indian-American? See what I mean?
Don't get me wrong, speaking for myself, 90% of the time I can be aware of the heteronormative (or other norming) going on in a movie but still totally enjoy the movie for what it is. (Hell, I read romance novels, you want to talk about heteronormative AND gender-normative!) But sometimes when I've had a day or a week when it feels like the whole world has been shutting me out or slapping me around, I want to see a movie with people like ME in it, dealing with the shit I've been dealing with. I didn't feel much like watching happy joy joy romantic comedies right after Prop 8 passed, for example.
I hope I've explained this with some kind of clarity, and nothing I said implies ANY judgement of anyone who is inside the "norm" boundaries for who/what they are, or for enjoying those movies, or for not noticing what's missing when a movie is packaged with the assumption that they are the main or only audience to please.
I am also giving my interpretation of what Faceboy said and am not presuming to speak for him directly. I'm sure he'll correct me if I'm wrong in my understanding of what he was getting at!