If same-sex marriage is clearly guaranteed by the Constitution then the Courts should be able to declare it to be so, rather than needing to rely on legislatures and direct democracy, but that begs the question as to whether it is.
The Constitution says that all people should have equal rights, thus how could this ruling not be constitutional? There's no good (or bad) argument out there to justify to classify people into first tier and second tier (something regretably Hitler and his socially darwinist allies believed in) as would have been the outcome if the court had ruled differently.