> Should we encourage abortions for single mothers because their child might not have a happy life?

Yes.

> Should we abort fetuses who have some disability?

If the parents are unwilling or unable to deal with it every day for the rest of their life, yes.

> I'm sure that any of those individuals, once born, would prefer to live regardless of circumstance.

That's too big an assumption. But, in any case, not being born, and not having a working brain capable of abstract thought and understanding of the situation, nor a way to express it, they don't get a vote.