what are we supposed to say again when christians ask us where we get our morality?

[Esquilax]

I am not sure this is even a correct statement; fish and reptiles are sentient but hardly seem capable of making moral decisions.

You're looking at it backwards, is the problem: "only sentient beings are capable of taking part in moral thought," is not the same statement as "all sentient beings do take part in moral thought." The former is a subset of the latter.

Also, if morality is merely defined as the well-being of sentient beings does that mean that you’d choose to save two fish over the life of one Human?

No, because there's a progression of complexity at play here; the one human is, at least within the terms I am able to detect, more sentient than the fish. That said, if given the option I'd still prefer it that nothing died.

Even if what you were saying was factually accurate it is a non-sequitur. The fact that a being is capable of doing something in no way necessitates that being ought to do that thing. The fact that sentient beings are the only beings capable of having a definition of morality in no way means we ought to have any such system. Why not define morality as whatever is best for myself? Whatever is most painful for others? Whatever is best for my children? It’s completely arbitrary without God.

Entirely untrue: if morality only affects sentient beings- and it does, I don't think anyone here is going to argue that rocks feel the consequences of moral actions- then their reactions to the stimuli those moral actions cause form a part of one's moral determinations. You dislike pain, pain in general serves no good purpose as it denotes physical injury, so therefore pain is generally bad for sentient beings, and that forms a cornerstone of our moral systems. And to cut you off from the inevitable "that's in human terms, not god's terms," god seems to recognize that pain is bad too, as that's a part of the punishment he inflicts upon humanity in Genesis.

I really don't get why you'd consider this a nonsequitur... well, I do, but if you'd been thinking of the argument on its own terms, rather than through the prism of "everything is meaningless without god," you wouldn't consider it that at all. My given premise is that, since only sentient beings are capable of observing and taking part in moral actions, morality concerns sentient beings exclusively, it just sort of naturally follows that the welfare of sentient beings becomes a component of morality, with those actions that allow them to flourish, so that there are continued sentient minds to take part in morality, being morally good. We survive by cooperating, our lives are immediately improved by our ability to band together and delegate tasks, which is why humans specifically benefit from not being psychopaths.

I do not think you actually believe this. Is it morally permissible to rape or kill someone who is unconscious since they are no longer sentient?

I'm not going to play games with you, Stat: the person who is unconscious isn't totally non-sentient to begin with, and there is a very reasonable expectation that they will return to full consciousness in the future, potentially even during the act you perpetrate on them. We both know this, I really can't imagine why you'd think this wouldn't be my answer.