Most people who oppose same-sex marriage claim it violates a long-established religious tradition or will somehow harm parenting and relationships. The U.S. Constitution does not empower government to protect traditions. There is no legal power enumerated in the Constitution for state or federal government to regulate marriage at all.
When you set aside your assumptions and think about it, you might agree a government license for a personal relationship makes little sense. Child custody issues, property disputes, and other legal agreements are already handled by courts regardless of marriage.
The idea that government should privilege mom-and-dad parenting can be negated by all the successful single-parent families in the nation, and by the failures of every abusive heterosexual parent. The argument that marriage should be restricted to heterosexual couples because they can procreate can be negated by the many marriages between child-free couples, infertile couples, and senior citizens marrying for love and companionship.
The argument that marriage should be restricted to "one man and one woman" on Biblical grounds can be negated by First Amendment protections. Churches, synagogues and other religious communities are free to bestow or deny a blessing or special status on participating couples regardless of what government does.
People must be treated equally by government regardless of the shape of their bodies. Whether they are same-sex or "traditional," marriage licenses don't have anything to do with who the applicants find attractive or what their future might hold. Government never asks about romance or parenting plans before issuing a marriage license.
Legal marriage provides many marriage-related benefits and services, including better tax rates, that even "civil union" couples cannot have, and government perpetrates sexual discrimination by giving preferential treatment to applicants based on sex (the applicants' body shapes). Government must always provide "equal protection of the laws" as required by the U.S. Constitution.
Erik Viker, Selinsgrove