1. Couples who are denied legal recognition of their unions are not prevented from making commitments to each other, getting married in a religious ceremony, throwing a wedding party, forming a household, having sex, or doing anything married couples do. There's absolutely no infringement of personal liberty whatsoever. That's because legal recognition is not an allowance but a positive government benefit (specifically, a pledge by the government to enforce a marriage contract) meant to buttress the religious institution of marriage and promote monogamy. 2. Extending legal recognition to gay couples while excluding polygamists is completely arbitrary and hence represents 'invidious discrimination.' Conversely, since the state has an interest in seeing children born to stable families, and since only heterosexual couples can produce children, a line drawn at traditional marriage is justifiable. There seems to be no public interest whatsoever in promoting homosexual monogamy. Moreover, it seems probable that a civil conception of marriage which is completely divorced from the historical-religious nature of the institution would have about as much success in promoting monogamy as a government-issued friendship certificate would have in promoting friendship. Postscript: Gays don't care about marriage. They care about having the status of a protected minority class, so that anyone who disapproves of their behavior is branded a bigot. Gay marriage provides a faux civil rights movement, a vehicle for propaganda, and ultimately (upon success) a conferral of legitimacy by the courts. But it's not the real goal. The real goal is to make the social, legal, and financial costs so high that no one would dare utter a word against homosexuality.