Okay, you've got some issues here. First of all, people who cheat are not scum, they are horny and overtaken by emotion. More importantly, they are doing what nature programed them to do. They are doing God's (nature if you don't believe) will. Who are you to judge them?
Why do people bother to get married? Because society insists we do it to have children! End of story.
Haven't you guys seen the discovery channel? Sorry, but some of these statements are just plain ignorant. For example, there was a show about these chimps that stay with their mates forever. And why do they do this when humans cannot, given we're so alike. Know what they found? The female chimps were sneaking out at night and having sex with other chimps in other villages/groups.
Nothing is forever. As soon as you figure that out your pain will be gone. The more you resist the truth the more pain you will actualize.
You have anger issues as well. People will not "some day...get theirs" for doing what nature intended them to do. Relax.