It makes evolutionary sense for males to like the look of the female behind. Consider the standard mating position for thousands of years for people was doggy style and the relative positioning of the bum when standing up to the vagina when bent over in that position. As for tits, a firm, full set of breasts generally means that they will be a superior child bearer (so guys who happen to like this were more likely to be attracted to females, who in turn passed on their genes better), I'm not sure if the bum has some particular quality like this too - However I do know that 'wide hips' at least is desired for this reason.