A husband and wife cannot by any contract with each other alter their legal relation, except as to property and except that they may agree, in writing, to an immediate separation and may make provision for the support of either of them and of their children during such separation.
Alteration Of Legal Relation By Contract -- Separation Agreement
Mont. Code Ann. § 40-2-303, under Power of Married Persons to Contract.
Mont. Code Ann. § 40-2-303