The short answer is we don't know but the current best guess is thousands of years ago in the middle-east. Marriage often has legal and financial implications.

The short answer is we don't know but the current best guess is thousands of years ago in the middle-east. Marriage often has legal and financial implications. Those who want homosexual marriages probably want to also benefit from those. There are also non-trivial symbolic reasons. In this context I am using marriage and civil unions synonymously.