Or, the difficulties could be around the fact that Bettina won't let Marc see his son. She's so bitter, even more so if she finds out that Marc and Kay are together, that she tries to legally keep him from having visitation rights. This would cause conflict, not only between Marc & Bettina, but Marc & Kay, as they try to get used to being together, at the same time having to deal with this outside problem. I don't know anything about German law in this matter.
But I think Bettina could feel so betrayed that she would go in this direction, at least until she had enough time to get over it, and realize that keeping her son away from his father is more of a punishment for the boy than for Marc. It could also be played out in a positive light, so it would end well.