People have purposes (Fyfe calls them desires, but I think purpose the more accurate term) and act to achieve them. So far, so good. However, I do not see (having read the e-book and Fyfe's general article on DU, and skimmed the FAQ) where he develops any calculus for the weighing of desires against each other that he bases the utilitarian part of the thesis on. Using easy examples like rape and child abuse just gives everyone the same bottom line to defend. Maybe Fyfe didn't build the argument in order to arrive at the "right" answers to those examples, but it's playing into that bias in his audience. I was entirely unsurprised when the author of the e-book went on to smuggle in some quite contentious claims unargued:

More practically, if we desire a stable economy and falsely believe that Libertarian strategies will deliver a stable economy, we may end up thwarting millions of desires instead of fulfilling them. If we desire good health and believe that New Age superstitions or religious prayers are more effective than scientific medicine, we may end up thwarting more desires than we fulfill. [p.41]

And no, "if" and "may" don't excuse this.