I remember in the 70's and 80's - pretty much ANY movie with a male and female adult as leads - there was GOING TO BE a sex scene, even if it was PG-13 type. Not always - but you knew when it was coming. About two thirds of the way into the movie - it usually only ran about a couple minutes and usually modest nudity. But they all had them. I suspect it was there to make sure it got a more adult rating, because a G or PG rated movie was never going to attract a crown.
Same with violence. Whole movie - action, but very little violence - and then the gore scene that makes the movie famous. Everyone remembers the head exploding in Scanners or the bellyburster in Alien.