To begin, I think a lot of people are asking, "um, what is moral relativism?"
However, I think the question that you are asking is (correct me if I am wrong): Why do Christians think there is an Absolute Morality, when it is usually Situation specific (ie, stealing your watch is different than a mother stealing soup to feed her kids at home.)
I believe that the reason is that it is scary to think that Morality or Truth is relative, or that they may not fully understand The Truth, or what God expects.
Someone posted something about asking if it was loving to ignore sin and the Gospel.
I proposed that Jesus NEVER focused on this or that sin, but rather, what one SHOULD do. The Goats and the Sheep were separated by their actions or inactions of loving kindness towards other people. Christ showed mercy to the tax collect who humbled himself before God while being a taxcollector, but did not justify the Pharisee, who obeyed the laws, did not cheat people out of money, nor was like the prostitute, but saw himself above others - in a word, without sin. So, the message of the bible, then, is not about sin, but about loving your neighbor as yourself, and in doing so, you love God. Jesus said to give more than you are asked, to walk 3 miles if asked to walk 1. Whatever you give will come back to you 3 fold, so don't worry about it. Buddhists call it karma. The only people that Christ really chided were those in the Church, the Pharisees, that enjoyed their status among men, who looked down upon others, who thought themselves better in God's eyes. Christ threatened that, and they hated him for it, so yes, it is sad that churches have turned to law and sin, like the Pharisees, rather than focusing on living in loving kindness, for being known for their love, generosity, humility, compassion, and mercy. That is a crime.
That is threatening to someone, like the Pharisees, who prefer to go to church, then look down on their neighbor, and think, "thank you, God, that I am not like THOSE people". It is threatening to realize that your understanding of what God wants may be completely off base. It is threatening to your own sense of superiority. It is threatening to your own map of reality, which Christ turned upside down, honoring the adultress, the prostitute, the tax collector, and the Samaritan, and humbling the Sadducee, the Pharisee, the Priest, and the Levite.
To someone who must "hold" the truth, it is threatening to think that they don't have it. However, there is a great Buddhist saying, "If someone says that he is seeking the Truth, listen to him eagerly. If someone claims to hold the truth, run far away."
Why? Because one who believes they hold the truth will refuse to be corrected, even if they are mistaken. No matter how much you show them a Jesus of mercy, they will give you a God of wrath. To change their idea of God, they must examine why they enjoy a God of Wrath, and that is usually because they feel condemnation for others, no mercy for others, and to quote Depeche Mode: Everybody's waiting for The Judgement Day, so they can go, "Told you so!"
It is also a threat to think about what Jesus or the Bible means.
For example: I am the Way, and the Truth, and the Life. No one comes to the Father but by me.
I was explained that only people who have Jesus as their Saviour come to the Father.
Muslim? Going to hell. Hindus? Where am I going and why am I in this handbasket?
However, Jesus said this when he was alive. It wasn't (*pending my upcoming crucifiction, and resurrection, and then your repenting of sins, asking me into your heart, being baptized. Void where restricted.)
"The Father" is everywhere, is a part of us (Buddhists call it "Buddha").
However, I believe that Jesus' "way" is to act Christ-like. Have mercy on others. Be kind to others. Humble yourself before everyone, whether they are homeless, a criminal, or on Jerry Springer. That is The Truth, and in that, creates the Life with God, being in congruence with God, who has always been in your heart even in the womb.
My neice said that she was at a U2 concert, where they chanted, "Christian, Muslim, Buddhist, Jew, All is good, All is true."
She said, "I'm sorry, but I just can't chant that."
I understand, but what was U2 Chanting it for? To seek God in the way that you can, and not kill other people that seek God in a different way.
It's unsettling to see people killing each other over "God". That must sadden God beyond words. Rather, if one humbles themself, humbles themself to the idea that maybe, just maybe, they are the one that is wrong, or that they believe in the same religion as their parents, even the same denomination, so it only makes sense that a Muslim or Hindu or Buddhist would do they same, they could live in coexistence and peace. isn't that what God wants, rather than, "I'm right, you're wrong?" Does he want us simply flexing our ego? Or does he rather want us to live our religion, rather than preach it? If God could become man, and wash our feet, can't we at least wash the feet of our fellow man?
Namaste (I bow to the divinity within you)