Well, two ideas, actually.
There are a lot of folks out there that don't know any gay people. Or if they do know them they don't know they are gay. The only images that they have are the media interpretations of the gay lifestyle. Movies from the 80's about the AIDS epidemic, Will & Grace reruns, and coverage from rallies and parades in San Francisco.
They have impressions of public and promiscuous sex. These people shouldn't have the right to marriage. Marriage is not public and promiscuous sex!!!! They have little exposure to long-term, committed relationships.
Then there are those that don't want to set a precedent.
If you allow same-gender marriage, then the polygamists will start in and ghod knows what all will want it next. Spousal benefits will get so complicated that employers won't want to offer them at all.
So, what do we do?
Educate, not hate.
I was a bit surprised (and pleased)at how matter-of-factly same-gender couples show up on the home improvement and real estate shows on HGTV.
No comment, no big deal, just "Jim and Ted bought this 1895 Victorian for a B&B and spent 12 years restoring it. Our expert appraiser will tell them if them made the right choices." That kind of thing.
It was within my life time that the first interracial kiss was broadcast on TV. Now, we are going to have an interracial president.
Sometimes it's slow.
But it happens.