FWIW The Census says in 1996 about 69% of kids lived with two parents in a traditional nuclear family: defined as two (self ID’ed) parents living together — they included stepparents and unmarried couples living together as Traditional in this survey. Obvioulsy this doesn’t get directly at out of wedlock births - it does hint tho. About 1/3 of kids are living in situation where the parents are no longer together – tho that doesn’t mean they weren’t married at the time of birth.
This U.S. Govt Census site on the Marital status of Women at first birth gives you a historical prerspective in the last century.
About 53 percent of first births which occurred between 1990 and 1994 to women 15 to 29 years old were either born out-of-wedlock or conceived before the women’s first marriage. About 60 years ago, only one of 6 births was born or conceived before marriage.
Again though these are first births not all births
The usefulness of these numbers re the OP? I’m not sure but they are as rock solid numbers as possible.