Harm is a relative term. What might be considered harmful to one person might not to another. There are different kinds of harm: physical, emotional, spiritual, financial, etc. Therefore, harm is a personal thing that is experienced and is a bit subjective. So, when we ask how gay marriage harms anyone, we have to look at more than just one aspect.
Marriage has been universally acknowledged throughout history as a legal contract between a man and a woman in which there is emotional and sexual fidelity, along with childrearing. But homosexual marriage would change this. Since marriage is also a moral issue, redefining marriage is redefining morals. Furthermore, marriage is an extremely wide-spread practice within any society and has many legal and moral issues attached to it. So, when marriage is redefined, the society is dramatically effected. Legalizing gay marriage means changing the laws of the land. The ramifications are vast and we are seeing the effects of homosexual legal "rights" affecting housing, education, the work place, medicine, the armed forces, adoption, religion, etc. Are all the changes good? That is hotly debated. But we have to ask, is it morally right to force all of society to adopt the morals of a minority? (See Statistics on the percentage of the population that are homosexual and lesbian)
So, how would gay marriage harm anyone? First, let's define harm. Harm is damage to a person physically, emotionally, mentally, spiritually, financially, morally, etc. The definition is obviously broad and subjective, and this is problematic. People experience harm in different ways.
Here is a list of ways in which gay marriage can bring harm.