The government discriminates against many groups for many reasons. 16 year olds can’t vote and 20 year olds can’t buy beer. You can’t run for Senate in California if you live in Kansas. Priests can’t be compelled to testify about confessions, but bartenders can. Bricker or one of the other lawyers can explain it better, but there are several different standards the court uses to determine if discrimination is allowed.
For example, racial discrimination, because of the history involved, falls under strict scrutiny. A law or policy can’t even look like it might be racially motivated without extremely compelling reasoning and must be as narrow and specific as possible. Gender discrimination falls further down spectrum with intermediate scrutiny. You need to show good cause and that means are related to that cause, but not that it is as narrow and specific as possible.
Unprotected classes fall under rational basis. For this all you have to do is show that the law is rationally related to some legitimate interest.
Rational basis is the level that Judge Walker used to reject Prop 8. I don’t want to bring up the polygamy thing again, but a rational basis for limiting marriage to one person is that adding multiple parties increases the complexity of marriage laws (divorce, inheritance, power of attorney, next of kin). That argument does not work for SSM. In fact the only way it really practically impacts governments is that they may have to change some pronouns on forms.
Separate thought: I am the only one that finds this challenge to Walker ironic? The argument is that being gay and potentially wanting to marry at some point makes him more biased than a straight person either in a marriage or potentially wanting to marry. But, by that logic no non-gay person has enough interest in the case to have standing.
Second separate thought: I wonder if the USSC would ever accept a case like this without standing. I am thinking of the way they accepted Roe v. Wade. Normally the Court will not hear a case if the outcome is no longer directly relevant (in this case it was too late for McCorvey to get an abortion by the time the appeal reached them), but the court decided to hear the case anyway since that would almost always be the case with an abortion case. Could they do something similar with standing? I don’t think they would need to since other states will defend their laws/amendments and I don’t think anyone would say a gay couple wanting to marry would not have standing.