One contentious topic (with very little actual impact on the country) is gay marriage. On one side, judges decide that gay marriage must be allowed. In response, amendments are proposed to define marriage as one man and one woman. Amendments prevent judges from supplanting the legislature, but largely ties the hands of future legislatures.
One argument often mentioned is that once marriage is redefined beyond its traditional definition, there's nothing stopping it from being changed to embrace even more combinations, particularly polyamory, but also marriage between close relatives. It's a slippery slope argument.
Don't believe in slippery slope arguments? Tell that to the people of New Jersey. From Eugene Volokh:
Consider how the decision relies on the enactment of past gay rights laws. The backers of such laws often argue that these laws do not create a slippery slope towards same-sex marriage or civil unions. Thus, for instance, an editorial in the Boston Globe, Oct. 15, 1989, at A30, said "[A proposed antidiscrimination law barring sexual orientation discrimination in credit, employment, insurance, public accommodation and housing] does not legalize 'gay marriage' or confer any right on homosexual, lesbian or unmarried heterosexual couples to 'domestic benefits.' Nor does passage of the bill put Massachusetts on a 'slippery slope' toward such rights." See also Phil Pitchford, Council Members Wary of Partner Registry, Riverside Press-Enterprise, Apr. 30, 1994, at B1, quoting Riverside Human Relations Commission member Kay Smith as saying that "[t]hose that truly have a problem with homosexuality will see [a domestic partnership proposal] as part of the 'slippery slope' [toward gay marriages] . . . . But, this legislation needs to be looked at on the face value of what it is, and it really does very little." And see the Editorial, A Vote Against Hate, Louisville Courier-J., Feb. 3, 1994, at 6A, rejecting as "arrant nonsense" the claim that a hate crime law "would lead to acceptance of gay marriages."
Yet the New Jersey Supreme Court's equal protection argument begins by citing such non-same-sex-marriage, non-civil-union gay rights laws (citations omitted):
In addressing plaintiffs’ claimed interest in equality of treatment, we begin with a retrospective look at the evolving expansion of rights to gays and lesbians in this State. Today, in New Jersey, it is just as unlawful to discriminate against individuals on the basis of sexual orientation as it is to discriminate against them on the basis of race, national origin, age, or sex. Over the last three decades, through judicial decisions and comprehensive legislative enactments, this State, step by step, has protected gay and lesbian individuals from discrimination on account of their sexual orientation.
The way I see it, from the beginning of society, marriage became defined as the relationship between a man and a woman for plainly obvious reasons. The couple protect each other, and they most likely produce children to support them when too old or sick to do so themselves (among other reasons). Now, not every couple is capable of having children, and when older couples want to marry, we don't stop them, because the definition of marriage has been set by tradition for so long.
"It's tradition" is a reason to support not changing the definition of marriage. It won't get a passing grade in a class on logical argumentation. But if you bypass tradition, there's really no reason to restrict marriage at all. Let people who want to care for each other, in any combination, get all the financial benefits we now provide to married couples. After all, it would be discriminatory to do otherwise.