Million, your whole view on existentialism is grossly flawed, and further more you rely on a slippery slope argument that is even more flawed. I'm guessing you must be in high school or something (maybe an introductory class to philosophy in college), and stumbled onto something you thought was neato.
First of all Morality doesn't come from the bible, we as a society leave out the nasty bits to make it more culturally acceptable. According to your god, the source of all virtue, if a woman is raped in the city she is to be executed, if a man is to sleep with both mother and daughter they are all to be set on fire, God demanded the execution of all women and children of foreign nations (except virgin women, they could be taken captive, forced to shave their heads, sent to the mountains for a month, and then forced into marriage, where if the man wasn't satisfied with her he could divorce her for no reason, unlike his other many wives who he could only divorce if they had sex with some one else, which would result in their death), cruel and unusual punishment was used for even the most minor of sins. Ask yourself if you'd be willing to beat one your your friends to death with rocks because he set a broken bone of one of his children on the sabbath. The hebrew god is one of the most horrifying figures ever written into literature. If you want the scriptures where he demands these things I will gladly look them up for you. Jesus through in some lovey dovey parts, but that doesn't excuse the atrocities he commited and demanded of his people.
Now that biblical morality is officially bunk, where does that leave us? Where does morality come from? Societies define morality as time goes on, each one having different definitions, some refusing morality outright. The Chinese group known as the legalists in I think around 500BC actually said that Ethics was a worm that gnawed at the state. They believe that laws did not exist for moral reasons, but merely to create order among the state, and that morality was not even a consideration. That was their way of defining morality.
Most developed western nations use a morality based on freedom and liberty for the individual. I believe that it was Plato (some one who gave us many ideas about ethics and morality) that said that humans were basically good, and that government should exist to allow people to thrive and become their best, rather than quash and control people who will only harm themselves and others as the legalists and other more harsh societies believed.
As such, the pursuit of liberty and happiness is the basis of US morality and most other developed nations. To that end things we try to give as many equal rights to individuals, and only hinder the rights that are damaging to others which would infringe on their liberty and right to happiness.
Morality is relative you are right. But even the bible's morality is grossly flawed, and useable within a society if you leave out the nasty bits. I don't think there is a person here willing to live under the mosaic law that your perfect god created himself. I'm not going to use some one that burns children alive and executes women for the sin of being raped as my moral compass. You can argue that pedophelia is ok, but few civilizations will agree.