Why? No one's giving any kind of solid reasoning for why it would be.
I did give reasoning. I refer you to the part of my post that states that moral values of each species evolved based on conditions present during each species biological, social, and intellectual development. This results in both species being hardwired to see things through the prism of a set of values that evolved over the course of each species's evolution. This also means said species are biased and not qualified to make judgements on what the other one's way of life should be like. Yes, I see baby eating as wrong, reprehensible, and revolting. But I do acknowledge that I base that attitude on a set of values that I didn't invent, but was inherited. This means that my own opinion is biased and I can no longer say - as ridiculous as it may sound - that baby eating is universally wrong for everyone, everywhere, at any time.
Sure, but that also means that you can't say it in any context. If you can't say that it's wrong for aliens to eat their babies, then you can't say that it's wrong for humans (humans other than you, that is) to eat babies either. If me and my babyeater friend are sitting down to have dinner, with a human baby on my plate and a babyeater baby on my friend's plate, then it would be utterly inconsistent for you to interrupt my dinner but not my friend's dinner just because I'm of the same species as you. If I think eating human babies is great and my friend thinks eating babyeater babies is great, then neither of us shares your set of values and for all intents and purposes we might both be equally alien to you except for our physiology. What would the moral values of the human species be in that case; for or against babyeating?
What you're saying is that your own opinion (and everyone else's opinion, too) is biased and because of that you can't tell someone who has different values than you what to do, which is sort of fine. However, you're then adding in an extra condition that the aforementioned only applies if the other guy is of a different species than you, regardless of how alien or similar to you they actually are.
That still doesn't make sense.
What would have happened if I grew up in a society where baby eating was not only acceptable but considered the basis of all good? Would my revulsion to it have remained the same? Of course not - there's no absolutes when it comes to moral values. If I grew up in baby eater society I would consider any alien civilization trying to force me to abandon my inherited set of values evil. Just as I see the Reapers evil and Commander Sheppard good. I'm sure the Reapers have their own perspective on the matter
So, I can accept that my moral values work for me and my culture, and will keep adhering to them and applying them within the confines of my own culture. I will not try to apply them to cultures I can't begin to understand and will rather spend my time working with things I am qualified to work with. I think every civilization should have the right to live the way they choose to as long as it doesn't threaten others. They should also have the right to defend their way of life by force if need be.
Sure, but that doesn't really work, as it doesn't give you answers in a lot of situations. For example:
What do you do when an isolated backwards religious cult, which just wants to be left alone to live life the way they like to, mutilates their unwilling children horribly? You must let them, as to do otherwise would be interfering in their business.
What do you do when a babyeater scientist, who likes the same music as you and wants to be your friend and really doesn't want to eat babies, defects and pleads for asylum and the other babyeaters would do horrible things to him if they caught him? You must refuse, as to do otherwise would be interfering in their business.
What do you do when an alien society with a population of 1 million is ruled by a single brutal dictator and all the rest 999999 are asking you to help them and you easily could? You must refuse, as to do otherwise would be interfering in their business.
There's an infinite amount of scenarios in which your proposed system breaks down, because
a clear line can't be drawn between someone either being or not being part of your culture, or even your species. It's entirely plausible that an alien could be closer to you culturally and philosophically than millions of humans are. Not in terms of table manners or what sort of art they enjoy, but in terms of what they think about issues like the one we're talking about, the important things.