I think the first thing to remember is that a very evolutionary level, we crave connections. We are pack animals. We want strong relationships and are loyal to them. That being said, I think the reason there is a decline in the amount of relationships has a lot to do with 1) the fact that in the western world we tend to be in a constant mode of "self-actualization" which basically means we are out for number one. a lot of times we get so self-absorbed that we don't let a lot of people in. 2) social media. No matter how popular it is, it isn't normal. We were meant to be around people, not images and texts representing them on a screen.
Monogamy to ME is NOT natural + I am eternally restless + I am VERY easily bored + there are so many great people in the world + I hate being told what to do + I hate power struggles + I am super selfish + I won't compromise with anyone, unless its work related