Spousal immunity is a legal rule that forbids courts from being able to force a spouse to testify against his or her partner in criminal cases. However, if a spouse would like to testify against his or her spouse, then he or she can do so. Spouses must be married during the trial in order for spousal immunity to be effective.
Courts grant spousal immunity for the purpose of upholding the sanctity of marriage. Forcing one spouse to testify against another can destroy marriages, and so the courts have allowed for the respect of this institution. However, this doesn't mean that spouses will always refuse to testify against their partners. On the contrary, spouses who are upset with their partners may see the trial as an opportunity to bring their spouse to justice. So, whether or not a spouse testifies really depends on the individual marriage and the situation.