Person A having an affair with Person B when Person B is a direct report or directly within Person A's power in the company is a disqualification for leadership. Full stop. It is an abuse and shows that Person A is willing to abuse their position for personal gain. They cannot be trusted any longer.
I've seen people date their bosses and/or subordinates, and it was not an abuse of power, they just happened to like each other.
You're entitled to your opinion, and I'm not saying I disagree, but you should be aware that it's just an opinion. You're stating it as if it's some sort of objective fact.
Many people could just say the exact opposite of your sentence, i.e. "not a disqualification" and "can still be trusted". Furthermore, These people could supply many historical examples of leaders who did similar things and went on to remain great/effective leaders of the things they were charged to lead.