Ron Paul joined six other Republican presidential candidates Thursday in refusing to sign the controversial Family Leader "marriage vow."
Paul adviser Jesse Benton told Politico that he would not sign the pledge in its current form.
So far, Michelle Bachmann and Rick Santorum are the only presidential candidates to have accepted the document.
By signing the pledge, Santorum and Bachmann vowed to remain faithful to their spouses and support a constitutional amendment banning same sex marriage, as well as agreeing that same sex marriage was akin to polygamy.
They both also promised to ban "all forms of pornography and prostitution, infanticide, abortion and other types of coercion or stolen innocence" and only appoint conservative judges.
Adding to the controversy surrounding the pledge, the original version of the document signed by Bachmann and Santorum claimed that African American children were in some respects better off under slavery than they are today.
"Slavery had a disastrous impact on African-American families, yet sadly a child born into slavery in 1860 was more likely to be raised by his mother and father in a two-parent household than was an African-American baby born after the election of the USA’s first African-American President," the document said.
The Family Leader, the Iowa-based Christian group behind the pledge, has since removed the slavery language from the vow.
The group is asking all candidates, including President Obama, to sign the pledge.
Former New Mexico Governor Gary Johnson was the first to reject the vow, saying it was "offensive to the principles of liberty and freedom on which this country was founded."
"This ‘pledge’ is nothing short of a promise to discriminate against everyone who makes a personal choice that doesn’t fit into a particular definition of virtue," Johnson said in a statement.
Mitt Romney rejected the pledge on Tuesday. A spokesperson claimed that while Romney "strongly supports traditional marriage," the oath "contained references and provisions that were undignified and inappropriate for a presidential campaign."
Tim Pawlenty, Newt Gingrich, Jon Huntsman, and Herman Cain have also declined to sign the pledge.