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.