It’s difficult to argue that bans on gay marriage are a form of sex discrimination because no one gender is being singled out for inferior treatment. However, it is a form of discrimination which relies upon the same assumptions about gender and gender roles which also lie behind more common and direct examples of sex discrimination. Attacks are gay marriage are thus an effort to defend traditional heterosexual gender roles and patriarchal conceptions of marriage.
Gays are the latest and most public example of people refusing to abide by traditional norms for gender roles and sexuality. Growing acceptance of homosexuality and homosexual relationships erodes traditional assumptions about male power, male roles, and the male sense of self.
Martha C. Nussbaum writes in Hiding from Humanity:
Gays and lesbians are a symbol, in much of the public imagination, for sex without reproduction, for the decoupling of marriage from commitment to raising a family in the traditional way, which has certainly been a male-dominated way. ...The connection between recognition of gay unions and the erosion of traditional marriage is that if sex is thought to be available outside of the marriage bond, women will have fewer incentives to embark upon marriage and child rearing, and may not wish to do so if marriage continues to be a largely patriarchal and unequal institution.
In much of Europe, the birth rate has been falling alarmingly, largely, it is thought, because women have other opportunities in life and are unwilling to enter unions that will work to their disadvantage. ...gay marriage is scary because it is a symbol of sex, and therefore women, eluding patriarchal control. This sort of anxiety about change that eludes control, and the loss of control over cherished values, can easily awaken narcissistic fear and aggression. ...the panic about gay marriage is at least in part a panic about women eluding male control.
Religious conservatives do not seek direct and personal control over the lives of all women or gays, but they do seek a return to a time when traditional social norms and religious doctrines had a stronger, more authoritative role in people’s lives. Since religious conservatives also want to be the interpreters and arbiters of religious morality, they would in effect assume indirect control over the rest of us. This is what advocates of gay marriage and gay liberation must oppose.