"Like a Boy"

Listening to Ciara's song "Like a Boy," I begin to wonder why we assume that men will disappoint us and that some women expect it. 

"What if I?...
Had a thing on the side?
Made you cry?
Would the rules change up?...
Or would they still apply?...
If I played you like a toy?...
Sometimes I wish I could act like a boy"

Why is it that men are expected to act like "boys?" We as a society tend to let men off the hook with a lot of things because "boys will be boys." Where did this expression come from, and why is there not a similar expression centered around women? I believe that men should have to take responsibility for their actions. 

http://www.youtube.com/watch?v=_HKH7Emy1SY