I'll admit that logic mystifies me. Opposing marriage equality for LGB people to protect your own traditional marriage seems like protecting your children's education by opposing equal education for other children. It doesn't help your own children and it harms others.
Furthermore, establishing a legal precedent that gives state-sanctioned preferential treatment for one religion or set of religious traditions-- even if that religious tradition is yours-- endangers all religious freedoms. This is because it places in the hands of the state a power it should not have-- the power to dictate religious belief, to sanction some faiths and oppose others.
One needs only look at the history of the shifting oppression of Catholics, then Protestants, then Catholics again in England, as well as the Colonial American religious persecution of Quakers and those of differing Christian beliefs by the Puritans, to see that belonging to some branch of Christianity does not guarantee freedom for Christians within Christian states. Go look at the online comments of Christians to any contentious issue within the modern-day church and how often they accuse those of a different opinion of not being "true Christians." Then imagine giving the power to decide whether your beliefs are orthodox to a government official.
One of the most contentious issues today is the definition of marriage. Marriage, by its nature, is an expression of a person's most profoundly-held beliefs. Atheists do not seek a full Catholic wedding mass. Catholics do not find a Muslim imam to marry them. Muslims don't find a Pagan high priestess to perform a hand-fasting. Those who marry outside their faith believe such unions are acceptable. Those marrying a person of another race or nation sincerely believe interracial and/or international marriages are not immoral. Likewise, anyone marrying a person of the same gender holds to a faith or belief that recognises such unions.
When we insist the state should only recognise some marriages, but not others, we are asking the state to dictate religious belief. We are trying to hand the state power which it cannot and should not wield if we wish to remain free to exercise our own religious beliefs. We are asking the equivalent of that stranger on the internet to decide if we are a "true Christian."
I'd much rather protect my traditional marriage by defending the rights of LGB people to marry according to their own beliefs, and not mine or anyone else's.