He probably had attachment difficulties due to his adoption and difficult relationship with his adoptive mother.
I can list dozens of guys I know personally who were raised by 2 loving parents who were/are dogs who love to sleep around and even cheat on their wives/girlfriends. Some of them do it with pros particularly at "massage parlors" and the like but some would go barhopping just as they did when unattached. They find sex boring with the same person always and like the excitement of new partners. Problems they encounter is sometimes the women end up wanting more than just a meaningless fling and then they start getting in trouble essentially having multiple girlfriends the same time and they end up miserable. I can't say I feel sorry for them when that happens I think they deserve it because you reap what you sow.
Sometimes the woman feels the same way and they have open relationships or even swing. All I can say about that is I guess if they both agree to it then at least it is not cheating behind their back which is worse. Some of the men think that cheating is better because the woman would be hurt to know they go to massage parlors and say as long as they know it doesn't hurt them but it still can hurt them if they catch something and pass it on. People love to rationalize their bad habits though...
Some people like monogamy while others can't stand it. Some like monogamy but cheat simply because unhappy and find someone new then divorce/break up to be monogamous with the new person. Back in the day when morals were taught and meant something people were criticized and this kept a lot of people from doing such. Today they are taught it is ok to have sex outside marriage anytime you want with anyone you want and swinging and orgies are ok and playing with the same sex is ok and basically morals have gone out the window so it is chaos.
There is a movie called the To Do List. I found it extremely stupid. A girl decided she wanted to become experienced sexually and made a list of various sexual things to try and checked off each one she tried. It was sex just to try each one and didn't really matter who she did it with. Her sister had sex basically with any guy she could find. The mother defended and encouraged them against a father who objected. Feminists said this was a great movie because it encourages girls to be as sexually free as men without the stigma of being called a tramp. It was like she was just a robot as she picked a guy to try something with.
http://www.imdb.com/title/tt1758795/I have little doubt that at Jeremy's school some of the guys had contests to see who slept with the most girls. Sex is a game for such people not about an emotional connection they are seeking.