Meanings often change with times. The meaning of a word changes when people use it in a new and different way. Take the word, “gay.” Fifty years ago, if a man told you he was gay, it would mean something quite different than what it means to people today.
That is because how people use words helps to determine new meanings.
Marriage is a word, but it is also a civil, legal, and religious institution. The legal and civil purpose for the institution of marriage is to protect and encourage the lifelong commitment between one man and one woman to raise a family. By its very nature, marriage is the institution designed for the creation and nurturing of children.