The idea of an open marriage is not new, and having multiple sex partners before settling down has also been around since way back. Young folks are curious and always find attraction in something new. Just think of when you first learned to drive, even going to the corner grocery store to pick up a pack of milk was a long way for you to walk. As you grow older, you find out that walking is preferred over spending half an hour to look for parking. With experience and age, things take different turn.
I am married and before passing judgements to sluts (sorry) I can share why I personally feel that it is not always true. Sure there are times that one finds someone attractive and fanaticize sleeping with that person, but those are just harmless innocent thoughts. If two people find comfort in each other, sex is not the primary force that keeps couples going and be in love for rest of their lives. Infact, a couple can have a great sex life, but there would be other areas where things don’t work out. So sex alone is not the only thing that keeps couples attracted towards one another. Sometimes, the sex part takes a backseat and other issues predominate. It all depends on the circumstance.
It is only natural that you get bored so easily at this stage in your life. May be in a few years you might change your outlook and look for other things in your acquaintances. You should also note that whether or not you realize it, they also might only have a temporary attraction towards you as well.