The 'Smash' actress - who is thought to have been separated from her husband Nick Cokas for six months - was pictured locking lips with father-of-three Michael Morris, a director on her now-cancelled TV show, earlier this month and she is reportedly "upset" by the transgression.
A source told People.com: "This is a very hard time for her. She's very upset.
"Katharine cares about Michael. But she still cares about Nick too. She didn't mean for anyone to be hurt."
Katharine's paramour is married to actress Mary McCormack, with whom he has three young daughters aged between two and nine.
It's reported that Mary learned of her husband's steamy clinch with the 'American Idol' star just 24 hours before the pictures surfaced and she responded by asking him to leave their marital home.
However, the couple were spotted together on Saturday and Michael was seen sporting his wedding ring.
Katharine met Michael on the set of musical drama 'Smash', which was canceled by NBC in May after two low-rated seasons.