Bullies pick on you as long as the benefit is greater than the cost. If you don't fight back and the school/parents don't provide effective punishment, the cost is zero, so the bully will pick on you whenever it's worth slightly more than nothing to him. If you make the cost "getting kicked in the junk", the bully is only going to pick on you if he thinks it's worth getting kicked in the junk, which is not very often.

You don't need to be stronger, or a better fighter, you just need to make fighting you painful enough that the pain outweighs the gain. This is one of the key principles of deterrence (even on an international scale) -- you don't need to be strong enough to win the fight, you just have to be strong/vicious enough to inflict enough pain that "not fighting" becomes a better option for your opponent.