For thousands of years, people believed: The Earth was flat
Conceptually that is an easy conclusion to draw. Do you think of where a road takes you in terms of where on the surface of a sphere or in terms of where on a flat surface, i.e. map? You know the Earth is round, but your mind takes the simpler approach, and still thinks in terms of a flat Earth. It was reasonable for those without the equipment to study the matter to conclude that the Earth was flat.
We were the center of the universe
We believed we were the center spiritually and physically? Are you claiming mankind does not still consider itself to be the center of the universe spiritually?
There were witches
Some claim to be witches still, and their numbers are increasing.
The sun revolved around the Earth
Isn't science a wonderful thing that it can prove physical relationships between objects in space.
This falls into the same category as your first observation.
Gay marriage will not harm society nearly as much as your protectionist, short-sighted beliefs.
Unfortunately, this is just your opinion. The first country to legalize homosexual marriage did so in 2001. Can you say with 100% certainty that Muslims who despise Western culture haven't become more hostile to us because of homosexual marriage? I can claim that homosexual marriage is one of the grievances used by Muslims to incite their terrorist attacks, and those have surely harmed society. Or do you like the Patriot Act and other laws passed to fight the War on Terror?