Depending on what type of marriage or relationship you have, for most people an emotional affair is an act of treason.
The pain of a ONS is horrendous but an emotional affair is a further breach in the defences of a marriage. A marriage is a team, a union, an alliance to further the benefit of your family unit, financially, emotionally and for health.
For most the pain arises by a partner allowing someone to fracture and open up those defences within the relationship. That partner has invited an enemy and allowed their partner to feel unsafe and scared.
The fear that many betrayed partners feel is because they realise they are married to a defector who has willingly invited an invader to cause pain and humiliation.
It is effectively an act of war within the marriage, they may minimise it to be a fracas, a disruption a disagreemnt but it is far more. You have swopped alligence and welcomed another person to effectively batter/beat or destroy your spouse in so many ways.
It's the most disloyal thing you can do within a marriage to not protect your partner. Just with any war, sometimes you can forgive the perpatator, but it may take many years and sometimes there are just not enough years left in the marriage for you to become on friendly terms again.
In my opinion once an emotional affair has occured there really is no way back, the cracks are there and also the fear of your other half inviting another foe to take you down.
It doesn't really matter how or why it ended or how marvellous the affair is, one friendship has been replaced by another and in one way or another the union has been weakened.
The onus is really on the married person to keep those boundaries or barricades up, but many like to think it's the intruders fault.