This is federal court and the case is not being tried on legalistic order of operation grounds that the state court argued(that Prop 8 was illegal for what it truly was supposed to be was a revision and not an amendment, and if it was a revision it should have been voted by the 2/3rds of the legislature before it could have been put on the ballot). The other argument made in state court was that prop 8 broke the separation of power between judicial and legislature.

This federal court case on the other hand is putting up completely other arguments. According to Olson and Boies Prop 8 isn't merely illegal (and that the order of operations were done correctly as was argued in state court with Strauss v Horton) no Prop 8 is unconstitutional according to the federal constitution. There argument is as follows

1) Marriage is a fundamental right, it helps gays as well as helping society.
2) There is no reason, no societal benefit in not allowing gays getting married (thus triggering the limited powers of the constitution, the constitution only grants limited powers if the constitution is to discriminate they must show a legitimate government interest in why the government has to discriminate.) Google Judicial Review and Rational Basis, Intermediate Scrutiny, and Strict Scrutiny.
3) That the only reason for the government to allow Prop 8 was Animus. If Animus is the only reason why the government would limit marriage to straight couples than it would be unconstitutional. This is really no different than Romer V Evans, a Colorado case where there was a state amendment that prevented any part of the state (aka city, county, or state) government (legislative, executive, or judicial) from recognizing homosexuals as a protected class. Well the Supreme Court struck down this amendment to the Colorado Constitution 14 years ago for there was no legitimate state interest in making this distinction and the reason for the amendment was Animus. The supreme court case struck down this Amendment with a 6-3 vote (Scalia, Thomas, and Rehnquist dissenting)