CK,
In all honesty,I do think you are trying to overanalyze your fiances' behavior at this stage.I can understand your desire to know why people cheat but in regards to your fiance,it's almost tantamount to asking why a murderer kills.There are no specific answers that can be stamped to each and every person.
I would like to see you focus on getting away from this man that you are not even married to yet.There are big red flags flying all over the place and although I have only said this maybe 4 times since I have been here,I think you should get away from this man,not go forward and try to "recover" what is barely even there.Not only do you have infidelity before you are even married,you have verbal abuse,emotional abuse,if not yet,physical abuse is surely around the corner and porn(casual sex and personal sites) sounds like it is a problem too.Your fiance risked your lives with an STD and maintains his poor behavior.I am concerned for the safety of your child as well as you and you do deserve better than what is being handed to you.We are not talking about a long term marriage that was happy at one time,you are not married and it sounds like you want to rescue him from himself but you can't.You cannot change him either like others have said.I can almost guarantee you that if you do marry this guy,all the problems you have now would not only still be there but things will escalate.
Please reconsider where your energies are going right now.You can be loved and find love again but love yourself FIRST.
DO NOT accept bad behavior just to have a man in your life.Protect that child.
O