UAFA would allow a US citizen to sponsor their same-sex partner for immigration to the US, a right that is currently denied because of DOMA. This is an important issue that affects 36,000 couples according to the U.S. Census.
Rest assured, this is not going to create a loophole that allows an open border for anyone to legally immigrate. Permanent partners would go thorough the same screening process that heterosexual married couples go through, and face the same penalties for fraud. In addition, the American partner would be required to sign an affidavit committing to support the foreign national partner for ten years, even if the partnership ends.
*This information was taken from the Immigration Equality Action Fund website however I'm not affiliated with this amazing organization.