I recently answered a question about whether or not a man (the alleged victim) could be forced to testify against against his wife who was being charged with aggravated assault. The husband did not want to pursue the case, but was subpoenaed by the state anyways. In answering this question, I referred to the Florida Statutes.
90.504 Husband-wife privilege. (1) A spouse has a privilege during and after the marital relationship to refuse to disclose, and to prevent another from disclosing, communications which were intended to be made in confidence between the spouses while they were husband and wife. (2) The privilege may be claimed by either spouse or by the guardian or conservator of a spouse. The authority of a spouse, or guardian or conservator of a spouse, to claim the privilege is presumed in the absence of contrary evidence. (3) There is no privilege under this section: (a) In a proceeding brought by or on behalf of one spouse against the other spouse. (b) In a criminal proceeding in which one spouse is charged with a crime committed at any time against the person or property of the other spouse, or the person or property of a child of either. (c) In a criminal proceeding in which the communication is offered in evidence by a defendant-spouse who is one of the spouses between whom the communication was made.
Although it seems counter-intuitive, the husband in this case could not refuse to testify against his own wife if subpoenaed by the state. He could always refuse to show up in court, or show up and refuse to testify, but then he would be subjecting himself to the risk of being found in contempt of court.
The marital privilege, therefore, only applies to statements made by one spouse to another during the course of their marriage. The privilege can be asserted by either spouse.