Correct if the court finds there is a fundamental right to gay marriage. The court in the past has found there is a fundamental right to marriage. A fundamental right to marriage != a fundamental right to gay marriage.

The most narrowly I can see the US supreme court throwing out "gay marriage bans" but still side with advocates for same sex marriage (instead of siding with advocated who are against same sex marriage.) There is no fundamental right to gay marriage, there is a fundamental right to marriage, treating gays and straights differently with marriage and creating "marriage-light" institutions for gays is unconstitutional not because there is a fundamental right for marriage but because the state hasn't show a rational basis interest for the discrimination between gays and straights. Thus if this interpenetration is implemented ironically states that have civil unions will be forced into gay marriage but states with blanket gay marriage bans may be allowed to keep them.

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Ironically the interpretation of the last paragraph was the one argued by the most recent US Federal Court of Appeals. (It was not done by the district court). The reason why the judge who wrote the majority opinion did so was that he wrote his opinion in the most narrow way possible. It is not his job to declare fundamental rights that is a job for people above his pay grade. Instead he wrote the decision in the most narrow of a way possible, using the logic of Romer v. Evans instead of Loving v. Virgina.