Those who fall in love with a married person get trapped. That is the trap of dishonesty.
Such relationships will never be happy. The married person is having dual commitments and is hiding his love from his/her spouse. What use is that marriage? The thought that I am cheating my spouse is enough to kill all the joy. Dishonesty never gives joy.
Even if the married person wants to have dual commitments, he/she should make it very clear to the spouse. There should be no hide and seek. Once you tell your spouse, your spouse has to decide about whether the relationship should continue or marriage be broken. Do not be surprised if the spouse the other relationship to continue for love of his/her partner.
As far as the unmarried lover is concerned, he/she should stop seeing the lover unless the married end is sorted out fully. Otherwise you will again have the guilt of cheating. Such relationships are heart breaking and need tremendous understanding. Me and my own, that is the principal of life for most of us. No body wants to share his /her spouse with others. It was not uncommon in earlier days. But today, the whole idea seems difficult to imagine. Love and relationship are not easy to live with. Our emotions and feelings can bring us pain at any time.