I would have to disagree with this as it is a very narrow point of view. There are many types of relationships out there that are successful but not traditional and what works for me won't work for him and won't work for her. Open relationships and exclusive relationships can both be successful as long as both participants are open and honest about what they want. Heck 10 years ago online dating was dubbed as weird.

You said he got involved with 2 marriages very early in his life so he never had time to date. This is something that everyone needs to experience - The thrill of new encounters, new personalities, not knowing what the next date will bring. Sewing your wild oats I believe is the expression. Also some people just are not inclined to be monogamous. Its just the way they are wired. You can't paint everyone with the same brush and the refusal to accept that there are people out there who's view of relationships are outside what is considered normal is one of the things that contributes to higher divorce rate.

People are realizing that they want to be happy but also that their idea of happy may not conform to everyone else's. This is important because in order to make someone happy you need to be happy yourself.

With that said if he is not done experiencing life and you give him an ultimatum then it could lead to another failed relationship for him and you. My advice would be to wait it out if you think he is worth it. If not then better to move on and look for happiness elsewhere. WHatever the decision then a talk with him would be a good idea so that you are both on the same page.