From the opinion (http://www.supremecourt.gov/opinions/12pdf/12-307_g2bh.pdf)
DOMA is unconstitutional as a deprivation of the equal liberty of persons that is protected by the Fifth Amendment. Pp. 13–26.
DOMA is unconstitutional as a deprivation of the equal liberty of persons that is protected by the Fifth Amendment. Pp. 13–26.
In other words, DOMA was quite possibly constitutional right up until any state passed laws explicitly granting protections and rights to same-sex marriages. At that point however, it was in direct conflict with state laws and in an area where state laws prevail.