From an erotic point of view it makes a lot of sense for a director to get the actors to do this. It shows the female star and the audience the 'money shot,' but then there's the whole 'throwing caution to the wind' aspect that's a real turn-on. It's like the guys could be safe, and greatly reduce the chance of the woman having an unplanned pregnancy, but then they discard that value altogether by shoving it back in. Of course, it was 1979-80. Certainly all of these actresses were on the pill, but as a viewer you ignore that and think, "Wow, these party guests are real Libertines!"
what's the point of pulling out, nutting then going back in. Shit you might as well bust in it because when you go back in the rest leaks out inside of her. ??♂️??♂️