A LOT of it comes down to gays going from wanting to have privacy in their bedrooms, to wanting 'tolerance' of their lifestyle in public to now demanding legal endorsement of it.
Not a minor point...and one that won't show up in the type of polling done on this issue. By a large margin, the majority of people are accepting of their gay family members, friends and co-workers. They're just not interested in the details. And this is where the militant gay activists weigh in. Their goal is to have elementary schools put on rainbow pride parades in your neighborhood with mandatory attendance. Anyone caught not throwing fabulous confetti gets sent to the Village People tolerance camp.
These votes are about a line in the sand...and are largely out of self defense. It's heterosexuals who just want to be left alone.