If Lacant decided to do a part two.
As I wrote I would like Marc to go after Kay and everything is going to be allright.
Trying to think of a little realistic part 2, I can image it would go like this:
I almost know for sure that his relationship with Bettina is over. That part of the story is finished. Of course Marc has to make arrangements with Bettina concerning their son. He has to put his life together. He has a lot emotional stuff to deal with. And all this will take a lot of time (like every divorce).
Concerning Marc/Kay: let's assume Marc really did love Kay (I almost know for sure). Although he broke up with Kay and asked him to leave, the moment he found out Kay left, was unexpected for Marc and he was heart broken. He broke up with Kay because he had decided he couldn't leave Bettina. But after he realized that this was not going to work anymore, he went to Kay's apartment. Why? That's another interesting question. What did he want from Kay? Probably he just wanted to see him, be with him, sex. But Kay left. Because he gave up hope. One thing is clear to me: Kay was done with secret meetings and just sex. He loved Marc and was ready to have a real relationship. I think Marc wasn't ready at all. So, even if Kay should have stayed, it wouldn't have worked at that time.
But, if I put myself in Marc's shoes, it would feel "unfinished". Kay left just at the moment Marc realized his feelings for Kay.
So what I think could happen is this: Marc can't let go and after a while he is going to find out where Kay moved to. He contacts him again.
Let's assume Kay still loves Marc (and of course he does). And Kay has to understand Marc's situation at the time he ended their relationship. After all, Kay knew Marc was bound, he has to understand Marc couldn't leave his family like that.
So they meet again. Going out together, get to know eachother better, more talking. All the stuff they didn't do in part 1. And find out if there is enough love for a real relationship.
Let's assume there is. I guess then everything depends on the question: is Marc going to follow his heart and does Kay have the patience to give him the time he needs. I think yes.
I know it's still a romantic part two but....I really think it's not totally impossible this could happen.
Anyway, I won't watch a part 2 without a happy ending for the two of them. Would be too much.