We don't know if Marc is gay. I don't think so, he had a sexual relationship with Bettina and probably with other women before her. The only thing we really know he felt (strongly) attracted to Kay. Does that mean he is bi?
I don't know if he is ever going to say I am gay, I think Marc doesn't know yet what/who he is. And maybe that is not important and the only thing that counts is the one he loves, man or woman. Kay or eventually someone else. I agree with you that Marc is not ready for a relationship with Kay, but the end was a little hopeful (as I see it) and that's why a part two would be interesting.
You said it better than my feeble attempt. I'll clarify..
We know from the roadside chase where Kay forces Marc off the road and confronts him, Marc rages "I am not gay. Got it?" while punching Kay in full denial. He also reinforces his sense of responsibility in this sequence with "I am a father now... Shall I just abandon them?" This "not gay" is reinforced again during his locker room fight / encounter with Gregor, Marc never says he's gay, nor does he openly say / admit he had a relationship with Kay. Watch it carefully. He asks Gregor if he is Jealous.. after Gregor
implies Marc and Kay "had something between you two..." Yes, Marc kissed Gregor to clearly provoke him but nothing more... Marc grew a set.. Then when we see Marc next he's is in the club alone where he does a casual pickup, but realizes he cannot follow thru with casual sex with a stranger - not Kay. Like Ennis's relationship with only Jack, Marc only appears attracted to Kay enough to have sex - no one else. There's a big difference internally between accepting and saying "I am gay" and engaging someone implying there was a "something between you two.." That's why I think Marc will not admit to being "gay" aka very similar to Ennis.. V.