This is a very good point that you raise here, but I've already addressed it. It was precisely to cut off this line of thinking before it even began that I discussed, in my previous entries in this debate, the impact of Judeo-Christian thought on modern, and especially Western, moral thinking. The horror you express at a practice like infanticide is good (from my perspective), but it is culturally conditioned. Women are fairly frequently stoned to death for adultery and related crimes in Muslim countries. I'm sure that you and I are both horrified at the thought of such a thing as this well, yet Muslims tend to express a bit of surprise at our shock! Due to their cultural conditioning, they view such a penalty as normal and acceptable; due to ours, we view it as abominable. Similarly, Western countries continually express outrage at the so-called "human rights" (another uniquely Western idea) violations committed by Asian countries such as China; Asians, however, because of a different cultural conditioning, view this outrage by the West with a great deal of perplexity. I could name dozens more examples, but this will suffice; these are, after all, basic facts known to any sociologist or anthropologist or, for that matter, anyone who reads the news.
The problem that atheists face, as we've seen in this debate, is that they have rejected the philosophical positions (namely, Judeo-Christian beliefs) that support these uniquely Western ethical views. Let's use an example to demonstrate this.
A Muslim converts to Buddhism. In so doing, of course, he rejects the existence of the Muslim conception of God, the divine inspiration of the Koran, and the prophethood of Muhammad, among other foundational Muslim beliefs. He wishes, however, to continue to make his wife wear a burqa each time she leaves the house and to stone women for adultery. Are these desires of his to maintain Muslim moral standards compatible or incompatible with his newly-adopted Buddhist worldview? Clearly they are incompatible, just as desiring to maintain Judeo-Christian moral standards which include concepts like a belief in the innate dignity and equality of all human beings is incompatible with the adoption of a naturalist worldview.
It's also worth noting that I, obviously, believe in an objective, transcendent personal force who acts as a moral arbiter and standard. You, however, do not believe in such a force. It is, therefore, not incompatible with my worldview to appeal to the insights which this force offers equally to all and which go beyond any cultural variation. It is, however, a significant step outside of your worldview to attempt to appeal to any transcendent, objective force. The purpose of my statements is to offer an internal critique of your worldview, demonstrating its inconsistencies (that is, after all, the point of this debate); it does not matter to our purposes here that my worldview differs at this point.
While you cannot appeal to any objective, transcendent standard or force, you can, as you have, appeal to a share human instinct. The problem with this, however, is, as we have seen, human instinct includes much more than you give credit for and much more than allows you to hold to the culturally conditioned morality you would like to hold to. Human instinct may include insights like those offered by the Logos, Tao, God, Cosmic Christ, or whatever name it is that we choose to give to the transcendent, objective force from whom we (in my worldview) derive our moral insights, including some semblence to love (though not real love), compassion, and empathy, but human instinct also includes hatred, anger, selfishness, and violence. My worldview offers a basis (the Logos/Tao/God/Cosmic Christ/etc.) upon which to base my decisions of which aspects of human instinct I choose to nurture and which I choose to avoid and attempt to destroy with myself. Your worldview offers you no such basis; all you have is human instinct with its empathy and its anger, its hatred and its love, its selfishness and its selflessness, and nature itself, with its limitless apathy and meaninglessness.
We come again to the question: upon what basis do we, in your worldview, choose empathy over anger, hatred over love, and selflessness over selfishness? In short, upon what standard do we base our choice between Mother Teresa and Adolph Hitler? At best, in the naturalist worldview -- your worldview -- the decision is an arbitrary matter of personal preference.