The dictionary (base definition) of "Marriage":
1c : the institution whereby individuals are joined in a marriage3: an intimate or close union
Based upon the 10th Amendment to the US Constitution, that is a decision to be made by the individual states. Each one needs to settle on its definition. Despite various religious-based arguments, it really comes down to each society defining it, by law, for itself. So far 31 of 50 states have voted to amend their constitutions to adopt the "one-man and one-woman" definition. 37 states have a statute, constitutional amendment, or both upholding that definition.