In response to:
The push for same-sex marriage is not about wanting legal benefits available to heterosexual couples"
No, that's exactly what most gay couples want.
"same-sex couples are largely able to make contractual arrangements to achieve those benefits, and in many states, civil unions equate legally with marriage."
Key word here is "largely", which means that it is NOT EQUAL. Any federal benefits are denied, forcing states to discriminate against one group of legally married citizens. And civil unions (or same sex marriages in states that allow it now) are not recognized in most other states.
Within the next few months, Justice Anthony Kennedy will likely rule that same-sex marriage is mandated by the Constitution of the United States. The ruling will offend both common sense and Constitutional law. But it will nonetheless become the law of the land. With it, states will be forced to recognize same-sex marriages; same-sex marriage will enter the public school lexicon; religious institutions will be forced to recognize same-sex marriages or lose their tax-exempt status. Religious Americans will be forced into violating their beliefs or facing legal consequences by the government. The First Amendment's guarantee of religious liberty will largely become...
No, that's exactly what most gay couples want"
It doesn't MATTER. Gay people do not owe anyone a justification as to WHY they want something they're entitled to. Black people didn't owe anyone an explanation as to WHY they wanted to be freed from slavery. They had the fundamental right to freedom, not granted by government, and it was being denied to them. That was corrected by the abolition of slavery. No, gays are not subject to the same level of discrimination, but the principle is precisely the same.