Are you saying it can be a moral good to 'manipulate people who you hold into contempt into sleeping with you' because everyone wins: they get the benefit of your (false) empathy and nice-guyness and you get sex?

We can go with that example. "Moral good" is a lofty term, in the example it certainly gives the guy utilons, the girl utilons, seems like a win-win to me. Where's the downside? Or are you thinking of some personified "honest truth never manipulate people to your benefit" avatar, an idea made flesh, who's crying in a corner?

You could argue on the basis of "spoiling the common good by furthering a society full of dishonest manipulation". But then again, you could do the same with non-vegetarians, or car-drivers, on similar grounds (common good). Personally, if I could optimize the world I would mandate honesty for everyone (minus myself), could you imagine?

Does that "people are people" paradigm mean it's "pretty sad and pretty horrible" to press a secret button you see -- but the other human does not --, the pressing of which will help both people involved? Because that's just inefficient.

Where's the benefit in losing optimizing power to adhere to some vague-cultural-norms-reified paradigm about "must not analyze human behavior and act accordingly"? You can of course value it for its own sake, but why should others?