Because, historically speaking, wholesome families are built around the institution of marriage. Legalizing gay marriage allows LGBT people to formally build the same familial units that have been the backbone of our civilization.
Why do you want to keep LGBT people on the fringes of society?
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There are plenty of heterosexual atheists, such as Richard Dawkins. Do you think they should be banned from marriage too?
LGBT lifestyle is based on deceit. Why do you want to legalize deceit? Why do you want to call two people that can not make children natrally from their 'joining' a family? They are not, they are just a poor substitute of the real thing.
There are a lot of hetero married couples that can't have children einstein.
The are the exception, not the rule. Many of them would like to have children, but can't do to health or physical problems. Much different than two people of the same sex pretending they are the same as two people of the opposite sex. Again, those relationships are 'usually' honest if they know, or if they don't, find out after they are married. The majority of marriages (one man, one woman), have children the green way (no scientist, doctors or extra electricity needed). A homosexual couple can not have children as mates. They must take (legally adopt) someone else's child or one of them must CHOOSE to copulate with the opposite sex to have a child (not being faithful to their 'lifestyle'), or use artificial means (test tubes, implantation, etc). To say they are the same is deceptive.