I'd imagine this is quite rare today, though it obviously wasn't in the 80s and 90s. Today it's probably mostly blacks and some religious olds in churches with this position (like the Catholic Church and United Methodist), so maybe about 20% total.
Barely a majority (abeit a rapidly increasing one) support gay marriage now, and a substantial majority opposed it only a few years ago. The issue has only become salient over the last few years, far too early for the number of gay marriage opponents to fall below that of death penalty supporters. People over 30 have been socialized to oppose gay marriage for several decades of their lives, the numbers can't reasonably be expected to change
that drastically practically overnight.
