Men are men. Irrelavant people in their lives probably would not care enough to even ask. Men in general don't "feel bad" for anyone. There are exceptions but very few. Just proves, if you think he should "feel bad"since his girlfriend is "proud" to have him he is not "in" the relationship...just "showing up" for the relationship.
So,"why" you ask...simple ,,,,, " Consquences" result "IF" if people are both "in" a relationship dealing with every issue on the face of the Earth....money,time,etc. Men have a 'different' brain from woman and as long as they can maintain a 'distance' and 'deny' emotionally they desire another human,in their minds...they are FREE...free to answer only to themselves if the "direction of the wind" changes and a better woman floats by. Face it men who do this "LIE" to save themselves. Unless and until there is "something in it" for this type of man he will always lie about anything that suits him. He is number one and he can and will justify "anything...anywhere" that benefits him at the exclusion of others.
If you are the woman mentioned here...RUN in the opposite direction. Think about this question...How can a man go out after a quarter of a century of marriage to a woman who supported him while in college...took care of him...loved him...had his children...etc.etc....and pick-up a bar whore trolling for married rich men and then announce to his wife "I need space"....then when his wife finds out ....DENY it and then deny it again.
Simple,it suits his needs. Answer to age old question...."why do men cheat"?...Answer...
"oppurtunity"....and cheat can mean...emotionally,physically, mentally or financially. The keyword here is "justify"....the means will always "justify" to men,(at least 99.9%),the end.
As woman who do have a 'different' brain'become more like men in our world the line will be erased and woman will be capable of the same.