The "repugnant conclusion"

There's been a fair bit of coverage of the controversy caused when Vox.com commissioned, and then declined to publish, this essay by Torbjorn Tannsjo.   Apparently, the editors went weak in the knees over the possibility that some readers of the essay might "misinterpret it as implying opposition to abortion rights and birth control, which . . . is a real concern.”  Ah, open-minded inquiry.   (Brian Leiter has more on the episode here.)

Tannsjo was asked, initially, to present (and in the essay he defends) the “the repugnant conclusion,” a "belief that asserts our moral duty to increase the population size because, according to the argument, more humans means more happiness."  I have to admit -- and I'm not inclined to think that moral questions reduce to hedonic-utility calculations -- that I don't understand why this conclusion is or should be "repugnant", even to those who reject it.  That is, why should the conclusion be repugnant (rather than the argument be "unsound," if it is?