Although Obama stated that he personally believes that gay men and lesbians should have the right to marry under the law, he also qualified this position in a way that is very important from a legal standpoint. Obama believes that states should have the power to decide this issue on their own.
Why is this qualification important from a legal standpoint? This qualification means that Obama's support of same-sex marriage is strictly personal - not legal (even though he speaks about it as a "right"). If states retain the authority to define marriage in opposite-sex terms, then gays and lesbians cannot claim a constitutional right to marry each other.
To understand this argument, consider the quintessential liberal setting in which this type of reasoning frequently appears: abortion. Many opponents of abortion argue that states should have the sole voice on this issue and that it is not a matter for the federal government or federal courts. Ron Paul and many other conservatives have embraced this very position. The Supreme Court, however, held in Roe v. Wade that the Fourteenth Amendment gives women the right to terminate a pregnancy. Although the Court has severely weakened Roe since it was decided, this right is still protected by the Constitution.
Because abortion is a constitutional right, the federal courts and Congress have the power to protect it. Article III of the Constitution gives the federal courts the authority to decide "cases or controversies" that involve questions of federal law -- including constitutional law. The Fourteenth Amendment gives Congress the express power to enforce the rights it contains.
Accordingly, when critics argue that abortion should remain solely within the purview of state law, they are effectively arguing that it should not qualify as a fundamental liberty interest protected by the Constitution. States do not have complete authority over constitutional rights. This fact is well established and supported by constitutional text.
Applying this same reasoning to marriage equality, the argument that states should have autonomy over the subject of marriage means that states could prohibit or permit same-sex marriage. The Constitution, which is federal law, would have no bearing on this decision.
Accordingly, Obama's new position on marriage equality implies that the Constitution does not or should not guarantee marital equality for gay men and lesbians. Instead, it is up to states to recognize this interest.
Unfortunately for people wishing to enter into a same-sex marriage, only six states and the District of Columbia currently allow them to do so. While advocates of same-sex marriage have loudly applauded Obama's new position, I implore them to read the fine print!