See, I wouldn't agree that libertarians are truly socially liberal. They might think they are, but they're not.
I am not sure i understand your point. I consider myself closest to a libertarian (social liberal and fiscally conservative). essentially a lot of social issues should not be regulated (or legislated) by the government (like abortion, gay marriage, etc.). Personally I am pro-life but vote pro-choice and i believe every person should be able to marry anyone they want "in the pursuit of happiness".
freedom of religion - the founding fathers were not all really "Christians" as the Right would like you to believe. They were Deism, Unitarian, Atheists...along with Catholics and Christians. They came to this country because of religious persecution (along with not wanting to pay taxes, or whatever). They did not want Religion in the Government.
“If I could conceive that the general government might ever be so administered as to render the liberty of conscience insecure, I beg you will be persuaded, that no one would be more zealous than myself to establish effectual barriers against the horrors of spiritual tyranny, and every species of religious persecution.”
~Founding Father George Washington, letter to the United Baptist Chamber of Virginia, May 1789
“The Government of the United States of America is not in any sense founded on the Christian religion.”
~1797 Treaty of Tripoli signed by Founding Father John Adams
“Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason, then that of blindfolded fear.”
~Founding Father Thomas Jefferson, letter to Peter Carr, August 10, 1787
“The civil government functions with complete success by the total separation of the Church from the State.”
~Founding Father James Madison, 1819, Writings, 8:432, quoted from Gene Garman, “Essays In Addition to America’s Real Religion”