You know, I've always found moral objectivists absolutely infuriating. A lot of this has to do with fundamentalists being big defenders of objective truths that cannot be known. And well, let's just say I'm not a fan of fundamentalists.
The thing with fundamentalists are that they are convinced, no sure that they are right. They will harp on about objective truths and how something can't be true for one person and not true for another. They'll attack the notion of agreeing to disagree, using some ridiculous analogy comparing disagreeing on religion to disagreeing that gravity exists or that the stove is hot. Basically, a lot of the arguments I hear for why their faith is right are based on proving the need for an objective truth.
And yeah, if God exists, that means the atheists are wrong, just like how if God doesn't exist, the theists are wrong. But here's the thing, we don't know if God exists or not. No matter what side of the fence you're on, you have to admit that if God does indeed exist, God doesn't make himself known to mankind. That doesn't rule out there being a God, but if you're going to claim that your objective truth exists, the onus is on you to defend it.
That's where fundamentalists fail. Most of the "proof" they have amount to flimsy apologetic arguments or personal antidotes of how Jesus changed their life. It's not as obvious as something like gravity or the sky being blue. It may be for the person with the spiritual experience, but from the outside looking in, I have no idea what you're talking about. Why should I make a life changing decision based on something that anyone can claim?
The same applies to moral objectivism, but even more so since no one can really claim that they have all the answers to every moral problem. The moral objectivist sees things in an extremely unhealthy black and white, with no shades of grey. Some go as far to claim that anyone who deviates even slightly from their moral code is some kind of relativist or anarchist, claiming that anyone who doesn't follow their rules to the letter are one step away from serial killing. So just like with fundamentalists, they think they have the truth and you either agree with them 100% or burn in hell for all eternity. But at the end of the day, they are only humans using arguments that anyone else can use to come up with a moral code.
But you know what? Extreme moral relativists piss me off just the same. I remember taking anthropology classes and hearing about how we can't say that any culture is right or wrong because we're looking at things from a "cultural" view. Yes, they have even said that we can't judge cultures for sacrificing their children because we can't see things from their point of view. And that's where I say screw moral relativism. Yes, we can't objectivily prove that sacrificing children is wrong, but we can make a damn good argument against it (based on the obvious suffering it causes). To not even try to determine if something is right or wrong is just as ignorant as claiming you have all the answers.
Okay, I rambled too long here. TLDR: Moral objectivists piss me off with their black and white, my way or the highway mentality. Moral relativists piss me off because they are willing to put human rights on the back burner for cultural traditions.