I don't wonder at him having a child with Cora when he was young, but at him agreeing to marry that woman's daughter when he's old, knowing that he slept with the mother (even if maybe he doesn't even know about the child), 'cause that's a bit fucked up.

True...unless she made him forget. By then Cora had her powers. She could have made him forget he ever new her before, let alone was intimate with her.