This article, and many related, made me formulate my definition of

Dissected, marriage is…

• …a relationship between people. The extent of plurality in the relationship is not an issue, and because of this, the definition does condone polygamy, or other related forms, providing the other conditions are met.
• …a relationship where each person maintains consensual, independent and unobligatory links with every other person. Consensual implies consent (both in the legal context and in the ordinary one). Independent means that a person’s relationship to a partner in the marriage does not depend on other partners. Unobligatory means a consent which is freely given and not based on external considerations. Also note that every one of the persons united by marriage should consent to every other person. See the examples for more detail.
• …for the welfare of progeny. This is a condition which can be, but generally should not be relaxed. The major purpose of a relationship of this nature to demand codification by society (imho) is to accord recognition to its children, and as such, the purpose of this kind of sanction is null and void without this clause.

Now, some scenarios to clarify things:

Suppose you are in a two-person heterosexual monogamous relationship (the commonest form of marriage on the planet). Suppose your spouse desires another person and maintains an active relationship with that person. Even with your consent, this kind of relationship cannot be a marriage between you three unless you and that person has a meaningful relationship too (this does not necessarily imply a sexual one). Since marriage also implies a responsibilty to progeny, all three of you would have to contribute to raising children.

Again, if you and your spouse have an “open marriage” (a term often associated with swinging, but generally only implying that lawbook adultery is not a consideration), and if you have regular sexual relationships with other people, this situation does not preclude a marriage by this definition, as long as you and your spouse have a meaningful relationship (even if not sexual). As such, two people who are not attracted to each other (sexually) can be meaningfully married.

Of course, interesting things happen when marriage becomes more plural. Language will have to rush to keep up. Co-husbands & co-wives anyone?