Because they want a guy for a while but don't want to make and investment in him. This one is the worst case ever. He doesn't understand that if a girl is "easy" it may work for a while until she is ready o move on. Sleeping with another man's wife may get you some kind of pleasure but how can one ever develop trust is a woman who would cheat? Same with guys.
The girl he is with now is a drug addict and has had her children taken away by the state. She does nothing, no house cleaning, a job, nothing. She was pregnant we he met her and has taken the child as his but the mom is hellish to live with. His problem is he thinks with the wrong head. At 51 he has yet to be truly happy. That's because he never attaches to a woman who would commit wholly to their partner's best interests. They are interested in him and what he can offer them, but not interested in his happiness.
If you want to be happy in your relationship I would advise, don't go for guys like that and don't be a girl like that. I may sound old fashioned but my standards and boundaries kept me out of trouble, and the wife happy.
As for your "Mistake," you are single and without kids, you are entitled to that freedom. You'll recover and move on. I have someone close to whom that happened. Worst pain ever for her husband. And for her. Stay away from married men, they are the worst slugs on the planet, exceeded only by poachers who go after milf's. Unhappy married women who are mistakenly vulnerable. The bars are full of them.
And so are business/ work environments.
Going to bars is like trying to find a good apple on the ground. Low hanging fruit might be easy, but it is never as good as something for which you have to reach.