You’re stating your “definition” as though it’s fact. It’s not. You believe in one god from one trilogy of books that does not have any authority over a world that does not universally believe in that god. Languages change. Definitions evolve. “Marriage” doesn’t actually exist, it’s a type of social contract that people have agreed to make with one another. People were getting married before anybody had ever read or written the Bible, primarily as an economic strategy. To argue that your religion’s definition is particularly accurate is already an example of changing it from its original. You’re just sticking with yours now because you like it. If humans want to change the definition of a something that humans created in the first place (such as marriage), we’re free to do so. Modern dictionaries agree. Nowadays, people tend to get married as a way to express love and commitment. Gay people love each other and commit to each other, and therefore gay marriage is just as valid as any other kind.