I genuinely think that you are giving short thrift to the OP.
Well, at least we're no longer pretending a subclass of moral realists owns the market on the word "preference." By the end of the day I hope to be misrepresenting nobody!
If, as you say, it's all about having preferences and really caring about them, then sure, the OP is perfectly well within his rights to express his preferences. But in that case, he would be equally well within his rights to express his preference for coffee over tea, and we would be equally justified in ignoring him.
You're mixing systems in this paragraph. What is justification when it's all about preferences? Do you want to ignore him? Then do so. If you don't, then why bring it up? In neither case is a justification germane. If real justifications do exist, you still could have ignored him - I imagine many have. So what good is the justification?
Yes, ignore OP. Or no, don't. My preference is that you do, and I believe /u/dramaticperiods' response was not listening. It was, instead, pulling a page from the apologetic-of-the-month calendar off the wall and applying it to the nearest conversation; i.e. unless you appeal to an objective morality, then all actions are equally justifiable and we have no way of differentiating your view from that of a sociopath.
Is that really as far as you two understand moral anti-realism? Yes, once all justifications go away, nobody is left holding any justifications. The conversation begins there.
I think as religious people we should not ignore his criticism.
I don't think anyone is arguing that you should. Rather, I'm arguing that OP endeavored to explain why these things produce emotional responses in atheists (the title makes that clear) and at every point used clearly emotional, not moral, language. As a group we are often asked why we care so much about religion when we don't believe in it, and this is a straightforward answer. Namely:
If you christians ever wonder why we care so much about something we don't even believe in, why we always seem so angry and emotional all the time, why we scream and rant and yell at you. . . . consider this 12 year old dead girl the poster board for why.
OP is fairly clear: we are asked why we're sad and this is the answer. We can talk about emotional reactions to evil things without necessarily insisting that they're evil. I can rant all day about how oranges give me gas without once discussing their color, even if that's what you really want to talk about, what you think you have the vocabulary to better discuss.
So you have some moral opinions - neat. I don't see how they're related to what OP is saying. Even if we agree that 'child abuse is wrong' is an actual moral fact, that still doesn't explain why it produces such a response in OP. OP is about much more than insisting child abuse is wrong; if it stops there, no, you're not listening.
OP is not saying "child abuse is wrong." That's not really a conversation worth having, since most reasonable people will agree wholeheartedly in the first few sentences. OP is saying "shitty child abuse by religious folks is the reason we're so emotionally involved in religion."