A friend of mine turned into a bit of a femme fatale after a bad breakup.
She met this guy on a dating app, he had stated "non-monogamy" in his profile. They met and he explained that he had a girlfriend for ten years or so, but was not the monogamous type. He was always interested in other women, even with his first girlfriend.
His current long-term partner does not know that he had always been cheating on her, as he had someone else on the side when they got together she she took this very badly.
He would typically visit the women at home, stay for the evening, have sex and then leave.
My friend invited him to hers once, but then also asked to see his place. He invited her over during the day when both were off work and his girlfriend was at work, they had breakfast in their kitchen, sat and snogged in their lounge and then had sex in the bedroom, on the bed he shared with his girlfriend.
My friend then left and never contacted him again. She didn't feel bad, she laughed when she told me this.