Gains vs. Losses
Shaar haTefillah (Shaar Nipol): "And the person should consider the following account on his soul: What he gains when he engages in buying and selling in the markets against the loss he has when he looks at forbidden things there and the evil thoughts he will have thereby. Even if he makes millions, all the money in the world, will it suffice to compensate for the loss of transgressing only one sin?"
The Chida (De'vash le Fi): "With the eyes he becomes a servant of the Yetzer haRa to look into all the forbidden and vain places in the world, and through them he will lose the possibility of enjoying the pleasantness of G-d in the future".
Chatam Sofer (Chaye Sarah): "The Jews, they are holy and should not stumble through looking at women, G-d forbid, for all who do thus, are lost in this world and in the World to Come.