Why do girls always say this?

Almost all my life, every once in a while, I hear some girl saying "All men are the same." or "Men never change." "All men are chesters" etc. you probably get the point, this annoys me so much, since I was a little kid I have sometimes even felt ashamed of being a boy from hearing this (In a non-homosexual manner) honestly, I have been with my girlfriend since 7th grade, I am now in 12th grade graduating this year. Never in our time of being together have I ever even considered cheating on her with another female. The thought of it right now just disgusts me completely. I plan on marrying her completely, we haven't faced any problems of loss of connection or of any sort whatsoever (other than little tiny fibs but I quickly apologize to her as she has all my respect as my girlfriend and a woman and we leave it all behind) Anywho, since from hearing those phrases so much.. I always have that thought in the back of my mind "What if its true..that every man is the same?" and well.. I was just wondering.. what are your thoughts on all men being the same and being cheaters etc.?/: I for one, hopefully will never think about it.. all these things just keep making me feel ashamed of being born a man.. I feel like i'm not worthy of anything if I am to be considered a cheater and being the same as any other man.