Jenny Teeson was in the middle of a divorce when she found a video of her husband sexually assaulting her in her sleep. The discovery turned her into a leading advocate for overturning archaic laws that can make it nearly impossible to prosecute men for marital rape.

The footage appeared to have been shot two years earlier, on Jan. 1, 2015, hours after the couple had hosted a New Year’s party at their Minnesota home. It showed her husband forcibly penetrating her with an object as she slept in their bed, their 4-year-old son asleep beside her, according to a criminal complaint. She was motionless as he attacked her. She believes she had been drugged.

Ms. Teeson, now 39 , told the police about the video when she found it. But under a state law known as the “voluntary relationship defense,” some of the most serious sex crimes charges cannot be brought against an accuser’s spouse or domestic partner. Her husband ended up being charged with a misdemeanor, not a felony.

While marital rape has been a crime in all 50 states since 1993 — a milestone achieved after years of dogged campaigning by women’s rights activists — the overwhelming majority of states still have loopholes on the books that can make it difficult, or even impossible, to prosecute such cases, according to data compiled by AEquitas, a nonprofit in Washington that assists prosecutors in gender-based violence cases.