And I agree that we should treat animals better. I'm vegetarian. ...

However, I don't feel that saying that "we should protect babies because one day they will be human" requires aggregate utilitarianism as opposed to average utilitarianism, which I in general prefer. Babies are already alive, and already experience things.

Ah, the fact that you're vegetarian is somewhat illuminating. The next questions, then: Do you think pigs should be weighted more strongly as babies in the moral calculus? If not, is it because babies are going to become people? If it is because babies are going to become people, why does that matter at all?

This argument has two functions. One is the literal meaning of "we should respect people's preferences". See discussion on the Everybody Draw Mohammed day. The other is that other people's strong moral preferences are some evidence towards the correct moral path.

Agreed, but again, it's very weak evidence.