We all know how the attraction to women makes no logical sense. It is part of our animal nature, given to us by G-d for the sole purpose of the perpetuation of mankind. As our sages said, "everyone knows that a woman is a barrel of excrement and that her mouth (allegorically) is full of blood, and yet all run after her". Therefore, if one wants to save himself from the fires of this desire, he needs to stay as far away as he can from that which awakens his animal instincts. This is why guarding the eyes is so important. For as long as one has not seen anything to awaken his instincts, his logic can rule, and he can use his mind to overcome the desire without waging a major battle. However, once he has let himself see something that awakens his animal instincts, his mind is no longer in control and there is nothing he can do to change his instincts besides putting up a difficult fight. It is similar to someone telling a child not to put his hand in fire and the child says, "why not, it looks so nice" and sticks his hand in. Obviously, he will give a cry and pull his hand out as fast as he can. There is nothing the child can do to prevent the pain once he has touched the fire, since a human being feels terrible pain from fire whether he understands it or not. But there is one thing we all can do, and that is - don't stick your hand in fire in the first place! It is the same with guarding the eyes.