To answer your question, Coolio.. The girl continued to date the guy because she was young and knew absolutely nothing about human relationships. And he was older, and devious and sneaky enough to brainwash her into believing that it is normal and even healthy for him to want other women. Also, she loved him and, due to being extremely inexperienced, hoped that he will change one day...
This was an historical example - the couple in question split up a long time ago.
I never saw this - lass why is it not normal and healthy to want other women? We are mammals, and therefore are all naturally polysexual and polyamorous to an extent. It's just society has trained us to think that this sorta behaviour is not acceptable (apart from you will never see the bigwigs giving a shit - it's for the layman this kinda social conditioning rule).
Just coz he is older and can manipulate her mind a bit does not mean it is bad - she would not let him do this if she was totally against it - it shows openmindedness, which is a valuable quality.
If I am going to make a commitment to someone, then we will make some rules together - and that is OK. But what if I don't want a monoamorous relationship? I need to find a mate who will accept what I do want, and that sometimes I will want to share some sexual intimacy with other people - variety is the spice of life, after all.
Fair enough if this isn't what you want, but you can't hope that someone will change their ways. You need to push it if you want to keep them, push their boundaries a lil, see where they are, and if they freak a bit, then work out together what they will not compromise on, and work out if you still wanna be seeing this person in a sexual way.
Like me for instance - I want to go into a relationship or marriage with my partner knowing that I look elsewhere, but not in her company. That I am attrracted to other people, but that right now she is the only person I want to spend my life with. I want her to
know and realize that I am a human and I am capable of making mistakes, and that I only have a certain amount of restraint - so I will try not to fuck anyone else when I am with them (unless it's agreed I/we can do that), but I won't feel guilty or bad if I do it - because it's natural. That being said - I would also talk, when making these boundaries, about if I did slip up whether they would want to know or not.
That way - you can control the situation as much as possible. You cannot control how you feel about something until it happens, but if you let things open up a little, and not be so rigid, then you most likely won't feel awful and hurt and angry and bitter if your partner sleeps with someone else and you're told/find out about it.
OP - I feel like I a m derailing somewhat BUT you haven't returned with a more illuminating post, so this should at least all shed light on what us males are like as a collective.
Peace