Is the norm? I just want to hear other views on this.?

Ok, I am almost 17 years old and I want to understand this. I think that , or any type of thought of another women crosses a man's mind is betrayal. I obviously know that I sound like a prude, but I really want to understand this. That is why I am on here in the first place. Im not a "hard-core" christian or anything I just think that a man should only want to see his loved ones body. My mom told me that I am thinking as if life is a fairytale, but has the world really come to visual ? What happened to commiting yourself when your married? Is really sticking to that? I have really strong views and opinions but I need girls and guys opinions on this. If you feel that it is betrayal or simply the norm. Please explain to me because I don't understand.