If one party does not agree, then it is not an agreement. If you are referring to common law marriage, both parties have to be in agreement that there is a marriage.
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Common law is not "applicable after a year in Iowa" if you are talking about the factors a court uses to determine whether there is a common law marriage. It is much more complicated then that.
I have no idea what you mean by a "cohabitation form" - is this some kind of domestic partnership agreement? If you are concerned about assets relative to a relationship that you believe could be interpreted as a common law marriage, you should consult with a family law attorney.
As for your question, if it is not agreed to by both parties (i.e., signed, or agreed to by some other evidence), then it will not be enforceable.
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