I was talking to a guy before when I was more naive and younger who would chat for a few days and then out of nowhere block me for ages and then come back with excuses and me being the fool carried on talking to him for him then to block me and repeat. I began to get suspicious, especially when he sent a selfie on a bed with flowery sort of covers and so did lots of digging to find his surname and search his facebook. I found out he was with someone, and even more shocking is around the time he was being inappropriate and chatting with me he proposed to his fiance. In my experience if you tell the woman, even if you send evidence, she doesn't wanna know.
Another guy I was talking to as purely friends but he wanted more with me, I knew he was married from the off but he painted the picture that he was unhappy in his marriage and she was unhappy and cheating on him, that they were only staying together for the sake of their children blah blah. He even sent me pictures of her wedding and engagement rings left on the bedside table saying along the lines of "look, she doesn't even want to wear her ring" when I later found out that actually his wife was in a job where it was handy (and safer) to take your ring off and leave it somewhere safe. He was very convincing about how unhappy they were and that she was unhappy and a player, but I found out they got married the year before so weren't so unhappy and still are very much together and have had another child.
Men who cheat usually are far from planning to leave their partner for you, whereas (and not trying to be sexist) us women tend to be gullible when men are being all romantic and sweet with us and believe he genuinely will leave them and don't love them, which then means they can have their cake and eat it.
As for people cheating knowlingly, I just think it's a terrible thing to do to someone and very selfish. It breaks people and makes them really insecure about themselves and in any future relationships, it's an awful thing to do to someone. Imagine how you'd feel if you were madly in love with someone and thought you were happy and had it all and really they'd been seeing another person behind your back and two timing you. Friends of mine that have been cheated on are really affected by it and paranoid and insecure in all relationships since, it really is a hurtful thing to do to someone and my heart goes out to those on here who've been cheated on. I know sometimes people cheat as they aren't getting sex or feel unloved or not getting any attention, but that partner then should speak with their partner or even leave in my view, I was always brought up to leave if you're unhappy. Don't hurt someone for your own greed and good.