HAZEL PARK, Mich. — On a snowy night in 2011, April DeBoer, Jayne Rowse and their three children were driving in their minivan down a rural road when a truck, attempting to pass another vehicle, came barreling toward them.

“At the last second, he swerved off the road and veered into a field,” Ms. DeBoer recalled. “I don’t think Jayne and I would have survived the impact. It was that moment, that realization, that we needed to get things in order.”

They figured they could draw up wills and assign custody of their children during a quick meeting with a lawyer. Instead, they are headed to the United States Supreme Court.

After talking to a lawyer in Detroit, Ms. Rowse and Ms. DeBoer were stunned to discover that, as a gay couple living in Michigan, they were unprotected under the law: Michigan does not allow two unmarried people to jointly adopt a child, so their children were technically adopted by a single parent, either Ms. Rowse, 50, or Ms. DeBoer, 43. Each parent legally had no claim to the children her partner had legally adopted.