Abortion and Preference Utilitarianism
I don't believe a fetus can indeed desire to be alive. But correct me if I'm wrong. And it seems wise to me to demarcate human life and personhood. Not doing this is speciesist as Cows are more like cognitively developed humans than fetuses yet we torture and slaughter them without a thought. .
I assume at the end of your post you refer the paradox of the heap. I wrote this somewhere else:
The need to define fetal personhood does not indeed lead to a paradox of the heap, as some might suggest; changes in predicates of potential personhood occur at specific points in fetal development, regardless of the fact that it remains genetically identical throughout its development. These predicates are not vague; they are quite specific. This applies if you grant that being human doesn't constitute being a person per say, but rather self-awareness, consciousness, viability, etc. Thus one can retain the belief that a fetus is not a person with a right to life.
And no, fulfilling preferences is not the same as being happy. Perhaps I work hard writing good poetry when really what would maximize my pleasure would be eating a chocolate bar. Both fulfill preferences, but one results in greater happiness.
As for the meat of your argument: according to you it must me wrong to use contraception because one is preventing a being with a valuable future from being born. The same goes for celibacy.