That math doesn’t make any sense. This chart says that 45% of republican voters are at least somewhat in favor of gay marriage. If we take your numbers and say even 53% of the total population is, then to get an average of 53% with republicans at 45%, we would need democrats to be at 61%, which certainly seems more than plausible (if we assume the population is split evenly by party for simplicity).
I don’t know where you got the idea that a chart showing 45% equates to “more than half”.