Why do religious people find it hard to accept that you don't need religion to have a good moral code?

I really actually know very few Christians who think like this. At most they would argue that any moral code without religious implications is ultimately egoist. This would be to say that moral decisions that seem altruist or selfless are actually, even in a roundabout way, self-serving. This is more philosophical, though, and really any more discussion on egoism and the argument that all ethical code is egoist is beyond the scope of this site.

My selected answer on this related question sums up why one might argue that you cannot be good without God. Basically, to the Christian, God defines good, not you. If he is not there to define it then you would simply define it for yourself making it a code you approve of, likely because it is easy for you to follow. Now everyone else has their own code too, therefore, good and morality become completely subjective and have no bearing on the whole. Therefore, no one is absolutely good; they are only subjectively good, which may or may not overlap with others.

Now if you get a group of like minded persons together and they all agree 'this' is what is good you still have the same issue. That group has no more right then anyone else to make moral laws and decrees, therefore, to one who objects, it is not valid, whether the group can enforce it or not.

Ultimately, the Christian who argues this must argue that because God is the creator (owner) and has the power to enforce His decrees He is the only one just to make those decrees. This combined with the 'subjective good' issue laid out above makes the argument very convincing.

This is not to say though that a Christian who thinks this thinks the world would fall into oblivion without God. That is really just opinion and conjecture.

I know this is more of an argument than an answer, but the question that you asked is based more on stereotype than mainstream belief within the church. I don't blame you because it's actually very common, especially in an age like today where the one-in-a-million weirdo can actually be heard by millions online.

Though you might not be interested in Scripture, the following is some of the most important verses concerning sin. It was written by a man named Paul that is respected as one of the most important people in the new testament. Paul was noted in the bible to have been one of the biggest persecutors of Christians - it is said that he was likely involved in the killings of many Christians before his conversion.
Also note that this was written after he was saved. When he speaks of his personal sin and his lack of control, he's speaking as a true Christian who had already received far more blessing than I would ever hope for.

Romans 7:14 says:

We know that the law is spiritual; but I am unspiritual, sold as a slave to sin. I do not understand what I do. For what I want to do I do not do, but what I hate I do. And if I do what I do not want to do, I agree that the law is good. As it is, it is no longer I myself who do it, but it is sin living in me. For I know that good itself does not dwell in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. For I do not do the good I want to do, but the evil I do not want to do—this I keep on doing. Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it.

So I find this law at work: Although I want to do good, evil is right there with me. For in my inner being I delight in God’s law; but I see another law at work in me, waging war against the law of my mind and making me a prisoner of the law of sin at work within me. What a wretched man I am! Who will rescue me from this body that is subject to death? Thanks be to God, who delivers me through Jesus Christ our Lord!

He is admitting that he is a sinner (not to be saved -he had already found salvation -but to hold himself accountable) and that he doesn't always do the right things that he knows is required of him, and that he often does that which is wrong even though he doesn't want to (presumably wants to at the time, but doesn't want to do such things in general). This is the testimony of a true Christian -an actual right hand man of Jesus, like few (if any) from his time on could ever claim.

Because "good" is either a transcendent qualifier or a personal one. If it is a personal one, then it becomes meaningless, as everyone would be perfectly "good" by their own personal moral codes. If, on the other hand, it is a transcendent one, then that "good" is measured against a higher standard.

The contention is that without religion, people don't really have a true basis for identifying "good" in the first place. This is often misunderstood as a contention that irreligious people can't be "good," but that's a misunderstanding; Most people would happily agree that irreligious people happen to be pretty "good" oftentimes, but it's almost in spite of themselves (and that's the point). The give-away is that when trying to establish how "good" they are, irreligious people tend to appeal to arbitrary standard with a certain odd dogma that their views don't justify.

For example, I might say I'm good because I'm faithful to my wife, or I haven't killed any one, or I give to charity, etc....but all these things are arbitrary, and none of them are inherently "good" if one rejects all manner of transcendent moral rule.

Incidentally, different cultures and sub-cultures have held such traits (as the ones that make me "good") with different levels of esteem and contempt, so even if someone appeals to something vague like "society," we still run into a problem that all societies, then, are on equal footing in terms of morality: by definition, they'd all be perfectly moral. Without a standard that's higher still then there's no way to judge one society (or, maybe, more specifically one of a particular society's tenets) as more moral than another.

I have not heard any Christian claim this in any academic setting. The most Christians usually say about morality in this context is that without a transcendent law giver no absolute morality can be derived and that without one we are left with either a regress into subjectivity or a complete denial of morality. Or in other words if God is dead than all questions of morality die with it.

None of which has any bearing on whether any specific person is morally good or not. Their may have been some random Christian lay man who thinks this but I really doubt that any person who defends Biblical morality in the public square would ever claim such a thing.

Its not necessary for a person to have a religion to have good moral codes.

Hammurabi (1792-1750 BC) was a Babylonian King who gave one of the First Law in the World. The law was very similar the Mosaic Law, "Eye for eye, tooth for tooth".

Exodus 21:24 Eye for eye, tooth for tooth, hand for hand, foot for foot, 25 burn for burn, wound for wound, bruise for bruise.

Every human is given the ability to decide what is right and wrong. Even the murderers kill others knowing that it is bad, but they just don't care.

Many religions give the idea that doing good deeds is the way to Heaven and doing bad things will lead you to Hell. Thus promoting the idea of Salvation by Work. Some religions even lay down obligations to be fulfilled in order to enter Heaven/Paradise.

Christianity has a different concept. The Bible teaches us the idea of Salvation by Grace.

Ephesians 2:8-9 (NIV) For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God — not by works, so that no one can boast.

Your good deeds will not open the gates of Heaven. Your bad deeds are also not the main reason why you should go to Hell. But Christians should do good deeds because we are saved by grace. Christians are called to be Holy, because God is Holy.

Romans 12:1 (NIV) Therefore, I urge you, brothers and sisters, in view of God’s mercy, to offer your bodies as a living sacrifice, holy and pleasing to God—this is your true and proper worship.

Christ died for our sins. He took all our punishment on the cross. When we remember how much God had done for us, we recieve the power to overcome sin and reflect Christ-like character.