http://blog.foreignpolicy.com/posts/2012/05/17/america_the_e...
There isn't a "US's stated reason for not signing". Instead, there's lot of "yeah that'd be great, we'll get on that later" from the various administrations, and what sounds like paranoid whining from religious conservatives.
http://en.wikipedia.org/wiki/US_ratification_of_the_Conventi...
> On 16 February 1995, Madeleine Albright, at the time the U.S. Ambassador to the United Nations, signed the Convention. However, though generally supportive of the Convention, President Bill Clinton did not submit it to the Senate. Likewise, President Bush did not submit the Convention to the Senate. President Barack Obama has described the failure to ratify the Convention as 'embarrassing' and has promised to review this. The Obama administration has said that it intends to submit the Convention to the Senate, but there is no set timeline for it.