I think this is an intuitive reaction. It's like being grossed out by dog poop on the ground. You feel the disgust first, and although an evolutionary psychology explanation for that disgust may be true (eg, "you have evolved disgust for dog poop to prevent illness and infection) intellectualizing it further would be totally superfluous. People experience the proximate cause for these sort of evolved behaviors, even when they intellectually know better. If you think you're above this sort of reaction, try "choosing" to be attracted to an 80 year old woman.