For this guy, a woman's breasts
isn't enough to feed his sexual lust. So what does he do? He decides to go for her bottom.
And there you have The Assman. This guy is so crazy about butts, he wishes that one day he could do a lot of things to them such as cuddling, spanking
, and so forth.
Compare Chubby Chaser
where a man lusts for overweight women, although the tropes can overlap if the chubby girl has a VERY big butt. Related to Male Gaze
. Pretty much the orchestrator of many songs about butts