Argument: Marriage is categorically about procreation despite exceptions
Susan M. Shell. "The liberal case against gay marriage." Public Interest. Summer 2004: "It is not that marriages are necessarily devoted to the having and rearing of children, nor that infertility need be an impediment to marriage (as is still the case for some religious groups). This country has never legally insisted that the existence of marriage depends upon "consummation" in a potentially procreative act. It is, rather, that marriage, in all the diversity of its forms, draws on a model of partnership rooted in human generation. But for that fact, marriages would be indistinguishable from partnerships of a variety of kinds. The peculiar intimacy, reciprocity, and relative permanence of marriage reflect a genealogy that is more than merely historical."
[...] The deeper phenomenal differences between heterosexual and homosexual relations are hard to specify precisely. Still, these differences seem sufficiently clear to prohibit gay marriage without denying gays equal protection under the laws. Gay relations bear a less direct relation to the generative act in its full psychological and cultural complexity than relations between heterosexual partners, even when age, individual preference, or medical anomaly impede fertility. Gay relations have a plasticity of form, an independence from natural generation, for which they are sometimes praised, but which, in any case, also differentiates them from their heterosexual counterparts. No heterosexual couples have such freedom from the facts of generation, which they can limit and control in a variety of ways but can never altogether ignore. Intimate heterosexual partners realize that they might generate a child together, or might once have done so. This colors and shapes the nature of their union in ways that homosexual love can imitate, and possibly even transcend, but cannot share in fully.
Such considerations, and others like them, suffice to sustain the "reasonableness" of a legal distinction between heterosexual marriage and forms of gay civil union that might perform many of marriage's tasks. It is neither irrational nor necessarily offensive to deem gay unions significantly less like a generative heterosexual union than is a marriage between infertile heterosexual partners."