I should say that I always assume good will and I know most people who support gay marriage do not want to hurt marriage.
But it is also true that some of the architects and advocates of gay marriage are interested in precisely that: overthrowing what they see as an archaic institution.
Judith Stacey, for example is a sociologist, who was asked to testify as an expert witness in favor of SSM . This is what she wrote several years ago about what gay marriage will mean for marriage:
"Legitimizing gay and lesbian marriages would promote a democratic, pluralist expansion of the meaning, practice, and politics of family life in the United States, helping to supplant the destructive sanctity of The Family with respect for diverse and vibrant families. . . . If we begin to value the meaning and quality of intimate bonds over their customary forms, people might devise marriage and kinship patterns to serve diverse needs. . . . Two friends might decide to “marry” without basing their bond on erotic or romantic attachment. . . . Or, more radical still, perhaps some might dare to question the dyadic limitations of Western marriage and seek some of the benefits of extended family life through small group marriages arranged to share resources, nurturance, and labor. After all, if it is true that “The Two-Parent Family is Better” than a single-parent family, as family-values crusaders proclaim, might not three-, four-, or more-parent families be better yet, as many utopian communards have long believed?" Judith Stacey, Gay and Lesbian Families: Queer Like Us, in All Our Families: New Policies for a New Century 117, 128-29 (Mary Ann Mason, Arlene Skolnick & Stephen D. Sugarman eds., Oxford U. Press 1998).
The academic literature is rife with such suggestions--from advocates of SSM. Of course when opponents of SSM bring this up, they get accused of a "parade of horribles" with no basis in logic. But both advocates and opponents of SSM see that something big has changed when marriage becomes a union of any two persons. Procreation and family structure are out.
What's left of marriage? The heart of marriage as a legal construct becomes a legal preference that adult sexual intimacy comes in twosome, for reasons no-one really makes clear except "tradition!"
Disconnected from its role in sustaining the family, fidelity, monogamy and marital sex itself become personal moral preferences, with little clear reason for being written into law.
Marriage becomes the way we stamp an official government Good Housekeeping Seal of Approval on how people have sex and intimacy.
Does that make sense to you? It makes no sense to me, and I'm not even a libertarian. (Although I once was: a Randian, as we called it, then. But that's a story for another day).