On the other hand, I can't see how "more preference = more happiness = more good/utility.
Okay, I looked up Practical Ethics
(Second edition, American paperback version
). Singer makes a distinction based on self-awareness and rationality, which, consistent with Locke, he sums up in the word "personhood". According to Singer, personhood makes the following difference in the utilitarian calculus: Persons can consciously anticipate future pleasures and pains and, and the anticipation gives them pleasure and pain in the present. By contrast, non-persons such as animals and embryos just feel as they go along, without any additional pleasure or pain caused by anticipation.
With these preliminaries out of the way, here is Singer's position on abortion: "My suggestion, then, is that we accord the life of a fetus no greater value than the life of a non-human animal at a similar level of rationality, self-consciousness, awareness, capacity to feel, etc. Since no fetus is a person, no fetus has the same claim to life as a person." (page 151) Note that this is not quite
as drastic as it sounds: In the preceding chapters, Singer spends several dozens of pages arguing for a radical upgrade in non-human animals' claim to life.
Singer also addresses the distinction between abortion and infanticide on the one hand and comatose people on the other. In his own words: "They [comatose human beings] are not self-conscious, rational, or autonomous, and so considerations of a right to life or respecting autonomy do not apply. If they have no experiences at all, and can never have any again, their lives have no intrinsic value. Their lives' journey has come to an end. They are biologically alive, but not geographically [...]
"There is one important respect in which these cases differ from disabled infants. In discussing infanticide in the final section of Chapter 6, I cited Bentham's comment that infanticide need not 'give the slightest inquietude to the most timid imagination'. This is because those old enough to be aware of the killing of disabled infants are necessarily outside the scope of the policy. This cannot be said of euthanasia applied to those who once were rational and self-conscious. So a possible objection to this form of euthanasia would be that it will lead to insecurity and fear among those who are not now, but might come to be, within its scope.
"This objection might be met by a procedure allowing those who do not wish to be subjected to non-voluntary euthanasia under any circumstances to register their refusal. Perhaps this would suffice; but perhaps it would not provide enough reassurance. If not, non-voluntary euthanasia would be justifiable only for those never capable of choosing to live or die."