companionate marriage

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A marriage based on the mutual consent and equality of the partners for the purpose of companionship rather than with the expectation of child-rearing or financial support.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A marriage in which the partners decide not to have children, and to reserve the right to divorce amicably with mutual consent
• noun anthropology A conception of marriage where complicity, communication and support between spouses are considered important, rather than purely material questions of inheritance etc.