Yes. This is where we see again the selfish side of Marc.. me, not really. that was cold. V.
Yes, it was kind of cold. But don't forget that Kay is a free man and Marc has a relationship and is a father-to-be. A huge responsibility. I think at this point Marc doesn't want to think about choices, leaving his family is no option yet. He wants everything the way it is now en he feels threatened by kay's question.
Also I think at this time Marc didn't realise how important Kay had become to him. Sometimes you just miss something/somebody when it's gone.
Maybe it was selfish, but understandable. Marc had so much to loose.
After the baby was born, he really wanted to make the best of it with Bettina and baby. It hated the way he handled this with Kay, but maybe this was the only way he could handle it. Telling Kay and himself that it was only an affair.
He had to experience himself that it didn't work anymore with Bettina. And that Kay wasn't just an affair.
Too late, but this is what happens so often in life.