I don't see a problem with it as long as both in the marriage are fine with it. It's not an uncommon thing in the kink community, especially with the whole hotwives, threesomes, cucks, swingers thing. It's more just seen as a fun activity. It all depends on the people, if you're the jealous type then don't bother, but some see and think sex is more a passionate embrace to share with your lover, others see it as just a fun side activity.
I've known both sides, I know couples with open relationships who are more than fine because the love stays between them, the sex is more just exploration fun. The relationships? still as strong as ever. I've also known others like one of my best friends recently slept with someone in a relationship, but she didn't find out until after. She was devastated and felt sick but mainly because she thought he was lovely and wanted to pursue more.
To call someone a piece of shit for getting involved with a married person isn't fair without the details. Just because their relationship isn't the same as your ideal one, the dynamics could be totally different, and if everyone involved is okay with it then why is that a problem to you? That being said if it's done behind the partner's back and without their knowledge then yes i'm completely against that. There are pornstars with marriages/relationships too but I guess they're all pieces of shit. Fucking sinners, may they burn in hell. God bless they need some Jesus in their lives.