How mundane life would be if we didn't have fantasies, daydreams, and even feelings that we cannot act on.

Higher life forms would not have these thoughts

How ordinary existance would be if we had to control all of our thoughts so that they are "correct".

Does looking and lusting after other girls somehow detract from your relationship with your soulmate?

YES!! I would feel terrible

  Would you feel terrible if you knew that she were attracted to other men because they are GOOD LOOKING, but chooses to be with you because she loves you and not those other brainless studs, and does not act on those other attractions?

A standard suggestion in a case like this, is to use feelings of attraction to other women to promote your feelings to your true love.  To imagine her from time to time as being someone from your gym, and thus give a boost to your feelings for your soulmate.  I say that's only one of about 800 coping strategies.  (And can result in awkward name changes at inappropriate times.)
 

I disagree. I find that lusting after other women BLATANTLY is too much.

You are experiencing so normal a response to your surrounding environment, that coping skills for this situation are called "maturity".  I liken this situation to food.  I have a big sister who is getting bigger.  Her dietary habits necessitate her eating all manner of junk food that is put out for her convenience at her work place.  She says

compulsive behaviors more complicated than that

, "What can I do, how can I stop eating junk food when it's right in front of me?"  What do YOU think she should do?  My answer is simple.  For goodness sakes, just don't eat it!  Maturity means never having to eat junk food (if you don't want to).  Do you see the parallel?