Yvain said:

You can end up utilitarian either because you're a psychopath and don't have the special moral module - in which case you default to general purpose reasoning - or because you're very philosophical and have a specific preference for determining moral questions by the same logic with which you determine everything else, thus deliberately overruling the special moral module.

I participate in utilitarian forum, and from that experience I would add to the quote above by saying that there are some people who encountered emotional arguments about "painism" or "speciesism" (e.g. arguments from Singer; Ryder, and the like) and followed those arguments to utilitarianism. I would expect that there are few people in this category as a percentage of the total population (in part because few people seriously study ethics of any kind and fewer still find their way to that end).