Hentai objectification vs instinct

The questionability of women portrayed as being used only for sex, and the human desire to use women only for sex.  Both go against our "civilized" mindset, yet are natural feelings.  Perhaps this conflict between what we see as lawfully or ethically right vs what we see as naturally or instinctively right, is what makes hentai so powerful a combination. 

In other words, how can this be wrong when it feels so right? 

It's biology mostly.  If you constantly have sexualized views of women and sex itself, it can create psychological addiction.  This can fuel more intense sexual desires aimed at adult anime/manga, while avoiding contact with real women, causing the human gene pool to lack diversity.