What gives a person the physical drive to look at forbidden images, and for that matter men's drive to look at women?
the same one as the drive for all sins - the ruach hatuma which gives a certain pleasure as the vilna gaon writes:
"in everything a person does, a ruach (spirit) is sent down from heaven , and helps him to repeat more things like these. And this spirit does not rest and quiet until the person repeats more things like this. The person then finds pleasure in doing these things, whether they are good (mitzvas) or evil (sins). And this is what is meant by 'sin brings sin and mitzva brings mitzva (pirkei avos 4:2)'" And the greater the sin, the greater the spirit, and the greater the lust to do more evil. And likewise for a great mitzva, a spirit from a very holy place comes down and the person lusts immensely for another mitzva. And because of this, he finds enormous pleasure in doing mitzvos. As is known, the greatest of all mitzvos is learning Torah, and the opposite is idle chatter and frivolity (letzanus) which is the opposite of Torah. Therefore, there is greater pleasure in idle chatter and frivolity than in all other sins, even though there is no physical benefit, because the ruach hatuma (evil spirit) is exceedingly great....but really, the pleasure in the holy spirit is greater than the evil spirit (sin) because it is a true pleasure" - commentary on Mishlei/Proverbs 1:23
looking at women in order to derive pleasure is a powerful sin as the talmud teaches (berachot 61a):
" 'from hand to hand he shall not escape from evil' (Mishlei 11:21), [this teaches] whoever counts out money from his hand to the hand of a woman so as to have an excuse for gazing at her, even if he possesses Torah and good deeds like Moses our teacher, shall not escape the punishment of Gehinom"