Actually what's "natural" is for man to spread his seed far and wide to preserve his bloodline as much as possible. Some married guys will say what they need to in front of their wives and some even convince themselves that they do in fact never think about other women... but that is actually what is "unnatural".

You may not agree, however our flesh is an animal that is wired by instinct. We are not better than the animals, simply different in that we are actually souls occupying these flesh creatures. Fallen... broken... instinctual.

It is not my place to determine what is particularly right or wrong. What I do know is most of the male characters in the Bible had multiple wives... and aside from accepted interpretations of text, there are no clear answers regarding monogamy... it's simply the practice we were told by the humans before us who foolishly assumed themselves to have any spiritual authority when in fact they do not. In the end you must do what you find to be correct, but there is no way of knowing beyond basic consent what is truly right and wrong.