A lot of my favorite gay-themed movies are adapted from plays. The Sum of Us and Beautiful Thing have already been mentioned (BT was my all-time favorite until BBM). I love two others that come from plays from the 70s and 80s: The Boys In The Band and Torch Song Trilogy.
I know BITB is seen as a kind of relic and full of stereotypes, but it's actually pretty good. The characters are pretty diverse, and it talks about a lot of different issues. The two characters in a relationship explore the intimacy and monogamy issue. Living in the closet and self hatred are major themes too, and I think it's pretty well acted.
When I was 18, I saw Torch Song Trilogy on Broadway and it was a real awakening. I really enjoyed following Arnold's story, and it also has monogamy as an issue as well as raising kids and gay identity. Whether you enjoy the movie depends on how much you like or don't like Harvey Fierstein, since it's pretty much his show. But the rest of the cast is quite good too, including Matthew Broderick and Anne Bancroft.
One other movie I Netflixed recently was A Very Natural Thing. Again, I was so surprised by this little known flick from the 70s and how it really looked at the challenges of making relationships work in a gay, urban setting. I guess all three movies have that in common, that they take place in NYC, and since I live here I guess I relate more. As Larry McMurtry pointed out that's another place that BBM is different in that it's in a rural setting.
I can't think of any others right now, but I love giving gay movies a try. So many turn out kind bleh, but the gems are worth checking out.
Juan