Why are people so... not smart?

I have yet to actually come to a conclussion on to how people are so close-minded that they would deny other perfectly fine, healthy, sane, humans the right to be married, and even come up with things like this: a href="http://i58.photobucket.com/albums/g276/pure_evil2010/Hahahaha.png" rel="nofollow"http://i58.photobucket.com/albums/g276/p…/a (It's a gay agenda >_>.) This kinda thing just boggles my mind to no end. Anyone have any insight on this?