What is so wrong with gay marriage?

There is no real good reason for gay marriage not to be legal. People say because of religious beliefs..the laws are not supposed to have anything to do with religion. Some say it's because it isn't natural and doesn't yield children...almost all animal species have homosexual members, plus the world is already over populated. Some even say because people will take advantage of it and just marry their friends or something...well don't you think that already happens? The only way to move forward in the world is to be more open minded and rely in common sense rather than customs.