Which is asking yourself honestly whether a judge personally opposed to gay marriage could have come to the opposite conclusion without intentionally misinterpreting or ignoring the law. In other words, whether the court case was a formality - whether the Supreme Court appeal that will no doubt happen at some point will be a formality. I think it's fairly clear that the answer is that yes, such a judge could have reached the opposite conclusion, and no, the Supreme Court appeal will not be a formality.

Before even looking at the opinion itself, there have been important judgement calls made by the judge that could have gone another way - I'm referring to the decision of which materials to review and to take into account.

And then in the opinion itself, most of the "findings of fact" are, obviously, factual to some degree, although on certain topics they present things as incontroversial fact that other judges might have been a bit more hesitant about (most notably number 70 where he claims there is no serious debate anymore about children raised by lesbian and gay couples being not different from those raised by straight couples). And there are a few that are seriously dubious (most particularly number 61 which claims that "Proposition 8 amends the California Constitution to codify distinct and unique roles for men and women in marriage". When he gets to discussing the Due Process matter, though, and delves into what the "core" of marriage has been historically and what it is now, surely there's a good bit of subjectivism in that. Obviously I agree with him, but still one can legitimately disagree with some of the statements there.

And then there's (p. 132-133) the matter of whether moral disapprobation of something is a sufficient reason for legislative action - he seems to take it for granted that it isn't, based on Lawrence v. Texas. The SC did indeed strike down Texas' sodomy laws as he says, but it was far from a unanimous decision, and Wikipedia says lower courts have declined to apply that decision too widely later on (for instance upholding an Alabama ban on sex toys). That doesn't mean Walker is wrong to use Lawrence to rule as he does, but it does mean it would've been possible to rule otherwise.