Then you need to lobby with your state legislature to change the law, because in YOUR state, it's legal.
Yeah, but that doesn't mean that it would have actually flied, even in 1975.
In the late 1950's Jerry Lee Lewis, a famous at the time country/rockabilly star married a distant 13 year old cousin. It was legal but it outraged the whole country and did serious damage to his career.
On Law and Order, Special Victims Unit just last week, they featured a show where a pedafile pastor attempted to marry his 13 year old victim in order to avoid prosecution, and failed dismally...and even in 1975 would have failed dismally.
In the middle ages marriages between unlikely matches happened all the time to cement alliances...even when those alliances could not possibly result in children.
In a handful of cultures today very young girls are married off to much older men.
However, I cannot imagine a scenario anywhere in the word today, or in 1975, where a 12 year old boy would be married off to a 17 year old girl....even if somehow that 12 year old boy got a 17 year old girl pregnant.
So, if someone is truly trying to write a work of fiction that would actually have a shot at commercial success...the whole premise fails.