The day you become a father, the protective instinct kicks in.  It's a father/provider thing that God built into we men.  My daughter is only 10-1/2 and I've been watching the boys in the neighborhood, at school and at church for the last 8 years, knowing that any of them are potential dates and suitors for my little lady.  
I would even be bold enough to say that if you weren't protective in such a way with your daughter, you are failing her as a father AND as a man.  In Stu Weber's excellent book "Four Pillars of a Man's Heart", he describes the four basic aspects of true manhood: King, Warrior, Mentor, Friend.  In the King role, he describes how the King is like a shepherd.  Fail to protect the flock and the wolves will pick them off, one or more at a time.  A true father is the same way.  With the way the media and Hollywood hyper-promotes promiscuity, I either have to choose to protect my daughter, or let those "wolves" devour her.  She's too precious a person for me to allow her to become the next victim of someone's undisciplined libido.