Let us be clear; marriage equality is an idea whose time has come. It will prevail. Those who oppose it will not be remembered kindly by history. There will always be forms of sexuality that others will find distasteful. Love and attraction between consenting adults of the same sex is no more or less perverted than love and attraction that transpire between straight couples. Relationships can be breathtakingly glorious, mindnumbingly tedious, or heartrendingly tenuous. Restricting the rights of people who express their sexuality differently can only serve to repress these practices temporarily, but the desire will remain.
Marriage is a right by virtue of equal protection under the law. It is fundamentally a privilege, not actionable by single people. We grant this privilege as a form of social engineering to promote stable family bonds for rearing children. Children of same-sex couples are no less deserving of a nurturing and stable home. We tolerate marriage as social engineering but restricting which couples are fit to marry is a level of government intrusion that should be unthinkable.
Slippery-slope arguments that predict legal recognition of polygamy are absurd. Marriage privileges monogamy over celibacy, polyamory or indiscriminate sex. Polygamy is not polyamory. Multiple wives in a polygamous relationship are not married to each other. Mutuality, a key characteristic of modern marriage, is lost on the slippery slope to polygamy. Gender inequality is also an issue in polygamy but not in same-sex marriage. Polyamory recognizes primary and secondary partners, not all of whom are attracted to all members of the group. Marriage is ill-equipped to deal with the complexities of multiple partners with differential statuses. Without mutuality, the character of marriage would be radically altered and might well be better understood if called by another name.
Near strangers can marry, as long as they are heterosexual and not prevented by local law because of kinship or age. There is no vetting of their relationship by meddlesome strangers. International marriages, in which the government certifies the authenticity of attraction as a condition of naturalization, is the notable exception. We tolerate this odd government intrusion to prevent fraudulent claims to citizenship based on acquired family ties. Imagine such vetting of polyamorous relationships. It illustrates the fundamental problem of assigning rights based on sexual attraction. As long as we recognize marriage as socially desirable, we must leave it to eligible couples to decide whether they are fit to marry.
Slippery-slope comparisons with bestiality are so absurd that they barely merit a serious response. Marriage equality opponents may understand that pets can't give consent, but they often can't resist making the analogy. Heterosexual and homosexual love are qualitatively different, just as love between people is qualitatively different from love between people and their pets? The analogy is as insulting as it is absurd. I'm sorry, but you can't imply by analogy that one or both parties in a same-sex union is subhuman and hide your intolerance behind an impersonal hypothetical.
The opposition to marriage equality is almost exclusively religious. There is no more consensus within or among religious groups on this issue than on other social issues. The Establishment Clause prevents making laws based on religious preference. Jehovah's Witnesses may deny children medical attention, but Child Protective Services may rightfully revoke their custodial rights as parents. Reasonable people recognize this as the legitimate interest of the State. The Establishment Clause demands that there be no legal preference for religion. Reason demands consistency, whether a religion is practiced by the majority or a minority.
Neither the secular nor the religious arguments against marriage equality stand up to critical scrutiny. I have no doubt that the religious faithful are sincere in their principles. They feel they cannot condone that which goes against their conscience. They would have us believe that their opposition isn't motivated by hate although it achieves the same effect. If your faith becomes an impediment to your humanity, it's time to question your priorities. If you still don't understand the issue, imagine asking my permission to marry the person of your choosing.