First, America was not founded as a Christian nation. God is not mentioned explicitly in either the Declaration of Independence, nor the Constitution of the United States (if memory serves). The "creator" is mentioned in the Declaration and Christians believe that to be God, but God is not mentioned by name. Also, I'll direct those who misguidedly believe America is a Christian nation to article 11 of the 1797 Treaty of Tripoli (I have written about this previously) which states in its entirety:
As the Government of the United States of America is not, in any sense, founded on the Christian religion; as it has in itself no character of enmity against the laws, religion, or tranquility, of Mussulmen; and, as the said States never entered into any war, or act of hostility against any Mahometan nation, it is declared by the parties, that no pretext arising from religious opinions, shall ever produce an interruption of the harmony existing between the two countries.
One of the founders was President when this treaty passed and signed it into law (and I'm sure some of them were in Congress as well). It is pretty hard to look at this and still think that America is explicitly a Christian nation. Then again, some people have a talent for not doing research and putting on the blinders when it comes to looking at facts.
Second, if you are going to recommend ignoring the Bible's support of slavery because society now sees it as wrong, should you not do the same regarding the Bible's (alleged) condemnation of homosexuality? More and more people are supporting LGBT equality, they see homosexuality as just another variant of humanity, and believe that homophobia is damaging to people. Thus, by Robertson's own logic, the Bible saying (allegedly) that homosexuality is wrong should be ignored.