From the comments: Kay was supposed to be hidden and mysterious. He just shows up in Marc's life. All the questions: how much time between the scenes, how often, how long, etc. wasn't important. The scenes showed the actual status, not the time it took.
But because we are shown so few scenes of them happy together (actually only 2, before it starts to go downhill), the relationship
itself is a mystery, and that’s frustrating for me. Were they really only happy twice? We see them more miserable than happy.
The only 2 times we see them when they really enjoy being together: at Kay’s apartment when he gives him the key, and even then Marc acts as if it’s no big deal (
dammit, Marc!), and at the club.
The scene when they are running, silent, is so hard for me to watch, you just know the happy times are over already.
Then it’s a matter of Marc backing away and coming back, and never once thinking how that affects Kay. When Kay asks if he’s ever going to tell Bettina, Mark leaves. He backs away when he hits Kay, and then comes back to him when they have the argument in Kay’s apartment. He backs away and comes back to him in the same minute, at the hospital when he closes him off, then asks to be kissed.
I know, I know; this is not a happily-ever-after movie, and I appreciate that, and I love it anyway. But can’t we know a little more about the good times? (they don’t even have to be naked

) The happy times they had together that made the bad times worth putting up with? (oh, man, bad grammar).
They can make all this up to me in part 2, when they go back and recap the whole affair, before going on to what happens, and if Marc goes after Kay (of course he does).