Why would a married man go to trips with another married woman?

I'm rephrasing the question I asked earlier. My soon to be ex denies having an affair with this married woman, a mother of 4, who was once his coworker. I have proofs of their affair and don't want him back in my life. But he is insisting they had nothing between them. Why would he go out to trips with her if he didn't have a relationship? I told him that I don't trust him and don't want him back in my life ever. Why doesn't he get my points?