For me, however, it doesn't seem all that far from someone saying "I'm a utilitarian but my intuition strongly tells me that people with characteristic X are more important than everyone else so I'm going to amend utilitarianism by giving greater weight to the welfare of X-men."
There is a huge difference between discriminatory favoritism, and valuing continued life over adding new people,
In discriminatory favoritism people have a property that makes them morally valuable (i.e the ability to have preferences, or to feel pleasure and pain). They also have an additional property that does not affect their morally valuable property in any significant way (i.e skin color, family relations). Discriminatory favoritism argues that this additional property means that the welfare of these people is less important, even though that additional property does not affect the morally valuable property in any way.
By contrast, in the case of valuing continuing life over creating new people, the additional property (nonexistance) that the new people have does have a significant effect on their morally significant property. Last I checked never having existed had a large effect on your ability to have preferences, and your ability to feel please and pain. If the person did exist in the past, or will exist in the future, that will change, but if they never existed, don't exist, and never will exist, then I think that is significant. Arguing that it shouldn't be is like arguing you shouldn't break a rock because "if the rock could think, it wouldn't want you to."
We can illustrate it further by thinking about individual preferences instead of people. If I become addicted to heroin I will have a huge desire to take heroin far stronger than all the desires I have now. This does not make me want to be addicted to heroin. At all. I do not care in the slightest that the heroin addicted me would have a strong desire for heroin. Because that desire does not exist and I intend to keep it that way. And I see nothing immoral about that.
does have a significant effect on their morally significant property.
But not in any absolute sense, just because this is consistent with your moral intuition.
Last I checked never having existed had a large effect on your ability to have preferences, and your ability to feel please and pain
Not relevant because we are considering bringing these people into existence at which point they will be able to experience pain and pleasure.
I do not care in the slightest that the heroin addicted me would have a strong desire for heroin.
Imagine you know t...
EDIT: Mestroyer was the first one to find a bug that breaks this idea. Only took a couple of hours, that's ethics for you. :)
In the last Stupid Questions Thread, solipsist asked
People raised valid points, such as ones about murder having generally bad effects on society, but most people probably have the intuition that murdering someone is bad even if the victim was a hermit whose death was never found out by anyone. It just occurred to me that the way to formalize this intuition would also solve more general problems with the way that the utility functions in utilitarianism (which I'll shorten to UFU from now on) behave.
Consider these commonly held intuitions: