Sexual abuse is just okay
So, in essence we have a 12-year-old boy having sex with an adult woman. This is OK by 1980s standards. Now, today, in 2017 this would be taboo, as in reality, though Josh, the Tom Hanks character "looks" thirty, he's really 12. So for a girl, she may "look" 18, but guys are arrested because she is 12. Sigh, what a great movie to show our young kids.
So go for it Dad, have sex with that 12-year-old, 'cuz they are really 30 at heart, and at their sexual peak.
Gotta love Penny Marshall.