Don't conflate different concepts. Rational basis review is a whole 'nother beast.
The ruling correctly states that, due to the nature of Proposition 8, strict scrutiny would normally be the appropriate level of review. It concludes, however, that strict scrutiny isn't needed because Proposition 8 doesn't even survive rational basis review, and goes through each proposed rationale:
1. Heterosexual marriage is traditional. As Judge Walker points out, this fails as a rational basis since "the state must have an interest apart from the fact of the tradition itself."
2. Overturning Proposition 8 would phase in a social change. But California has already, in the past, permitted same-sex marriages, so: "The evidence shows that allowing same-sex couples to marry will be simple for California to implement because it has already done so; no change need be phased in."
3. Opposite-sex parenting is better than same-sex parenting. This fails because California already permits same-sex couples to adopt and raise children, and because "Proposition 8 does not affect who can or should become a parent under California law."
4. Proposition 8 protects the free-speech rights of those who oppose same-sex marriage and want to teach their children to be opposed to it. But Proposition 8 is only concerned with permitting or forbidding certain marriages, not with allowing speech or education concerning marriage: "as a matter of law, Proposition 8 does not affect the rights of those opposed to homosexuality or to marriage for couples of the same sex."
5. Homosexual and heterosexual relationships are different, and must be classified differently. This fails because "same-sex and opposite-sex unions are, for all purposes relevant to California law, exactly the same."
There's also an alleged "catch-all" interest. I'll just quote that one in full:
Finally, proponents assert that Proposition 8 advances “[a]ny other conceivable legitimate interests identified by the parties, amici, or the court at any stage of the proceedings.” Doc #605 at 15. But proponents, amici and the court, despite ample opportunity and a full trial, have failed to identify any rational basis Proposition 8 could conceivably advance.
The important thing to remember in reading the above is that rational-basis review, while not particularly demanding, is not the same as no review. "I say there's a rational basis, therefore there is" doesn't hold up in court; you'll have to make an argument and it will have to actually make sense. The arguments advanced for a rational basis for Proposition 8 failed -- they either did not assert a valid basis for legislation or were contradicted by other established law or by Proposition 8 itself.