Which power allows same-sex marriage?

Answer 1

In most countries, the authority to legalize same-sex marriage is exercised by the legislature. In some countries, the judiciary may order the legalization of same-sex marriage. In some countries, the assent of the chief executive (e.g., president) is required for the legislature to legalize same-sex marriage.

In the United States, Australia, Mexico and many other countries, each individual state or province may legalize or not legalize same-sex marriage, although Federal Laws or the federal courts may either mandate that all states recognize same-sex marriages performed in another state (as has happened in Mexico), or may permit states to refuse to recognize out-of-state same-sex marriages (as has happened in the United States).