The congress doesn't pass laws regarding marriages. Yet. Not really true, IMO. Consider
http://thomas.loc.gov/cgi-bin/bdquery/z?d104:HR03396:@@@L Signed into law in 1996, The Defense of Marriage Act, seeks to define "marriage" as a marriage between one man and one woman, in an attempt to prevent gay people from getting, say, married in Hawaii and then getting the same protection as other married people. Unfortunately, it it seems to be in character with the "full faith and credit" clause (Article IV, Section 1) of the U.S. Constitution, which follows "And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof." The same rationale could be used in turning state-issued driver's licenses into National ID Cards, couldn't it? (me)