I'm not sure if I'd go as far as to say he was one-dimensional, but I do agree they could have fleshed him out a little more.
As for Alex sleeping with Justine, I thought it was to do with the theme of hiding who you are and suppressing yourself. I haven't properley thought about how it fits in, but I really want to see the film again and think about it a bit more. There's a lot of ways to read into it.
I definitely didn't get the impression that the message was "this gay man's life would improve by having a different orientation" but like I say I need to give it some more thought to process how it fits in with the rest of it. Not sure if I'd go as far as to say Alex is one dimensional, though I would have liked a bit more about him for sure.
But yes, great film. One of the best last shots I can think of.