Is there anything in the constitution or any American law that defines marriage?

Answer 1

The definition of marriage could be deduced from the different provisions of the Civil Law and articles of the Constitution on family relations.

Marriage is the contractual union between a man and woman to have a family covered by property relations that they chose.

Answer 2

There is nothing in the United States Constitution that defines marriage. Marriage is defined and regulated by the laws of each of the fifty states. For example, California law (Family Code Section 300) defines marriage as follows:

300. Marriage is a personal relation arising out of a civil contract between a man and a woman, to which the consent of the parties capable of making that contract is necessary. Consent alone does not constitute marriage. Consent must be followed by the issuance of a license and solemnization as authorized by this division, except as provided by Section 425 and Part 4 (commencing with Section 500).

Other states have similar definitions. It is of course a subject of substantial controversy whether marriage is restricted to a union of men and women, as opposed to same sex marriages. Whether that is permitted is a subject of individual state laws.