There can be a large element of 'wait and see' to it. Emotions and feelings oscilate between extremes of love and hate very often. I imagine that intimacy between a couple on the rocks could play a large part in healing and eventual reconciliation. My guess is that this is usually the desire for at least one of the troubled couple, and I don't see a huge problem with it unless one or both parties knows that it's just sex for the sake of sex and are inwardly committed to eventual divorce. That's where I think it becomes unethical.