To pass constitutional muster, racial discrimination had to survive "strict scrutiny" by the courts. Government had to demonstrate a compelling need for its regulations, show they would be effective and narrowly craft the rules so they didn't sweep more broadly than necessary. That same regime should apply when government discriminates based on gender preference.Of course Levy argues that marriage should have no role for the state, but as long as the state is involved, there is no justification to exclude GLBT people.
No compelling reason has been proffered for sanctioning heterosexual but not homosexual marriages. Nor is a ban on gay marriage a close fit for attaining the goals cited by proponents of such bans. If the goal, for example, is to strengthen the institution of marriage, a more effective step might be to bar no-fault divorce and premarital cohabitation. If the goal is to ensure procreation, then infertile and aged couples should be precluded from marriage.
It's worth remembering that in one of the early hearings of the Prop8 Federal case, the proponents were unable to define a harm to straight families in marriage equality.
"What is the harm to the procreation purpose you outlined of allowing same-sex couples to get married?" Walker asked.
"My answer is, I don't know. I don't know," Cooper [counsel for the proponents] answered.
The libertarian Orange County Register agrees :
Our position is that the State, as an institution, has no right to decide who can enjoy the institution of marriage, which predates the State by millennia. The ideal situation would be one in which people who wish to marry and religious institutions were free to make their own decisions about whether to undertake the obligations of marriage and to bless such unions. However, since the government has decided to license marriages and to promulgate a series of legal privileges and obligations that are extended to married couples, we support the right of same-sex couples to marry (though we would never force a church that opposed same-sex marriage to solemnize such a union), and opposed Prop. 8.