We have been over this hundreds of times. The issue is putting one's beliefs onto others.
The issue is pitting belief in a religion and morals against the deity of the self. There is no more obvious case of self-worship than that of demanding that reality be changed to fit with my whims. The founding fathers, as stated before, never intended for the country to be run on self worship.
ETA: To bring back a quote from earlier:
We have no government armed in power capable of contending in human passions unbridled by morality and religion. Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other.
-John Adams