What you've described is sex (for which you gain health) followed by violence. I'm familiar with both of those things, they've been in previous GTA games, but the point is that they're unrelated. For the sake of the video the player has chosen to do one right after the other, but they could have just walked away after the sex or committed the violence without the sex.
Your suggestion that sex should render the NPC invulnerable is... odd. Before the sex she's an NPC just like any other, after the sex she's an NPC just like any other.
All right, lets look at this another way: in Halo players have the ability to crouch, this serves a functional purpose. There's a rather juvenile tradition in Halo of killing another player in a multiplayer match and then standing over their corpse and crouching. The existence of the corpse and the ability to crouch are entirely separate from one another, each there for a good reason, but when the player decides to combine them in this way they do so with the intent to suggest a humiliating sexual act. There are ways that Bungie could prevent this one particular act if they chose to do so - they could eliminate corpses, they could make the areas around corpses impossible to crouch in, they could remove the ability to crouch entirely - but the act exists because the players wanted it and created it themselves. So in other words: 1) The fact that people use the game as a medium for their expression, and that expression in undesirable, does not mean that there's anything wrong with the game. 2) Any attempt to censor this sort of thing is likely to get worked around. 3) Free expression isn't always nice, it doesn't always make you feel good about humanity, but it is always valuable.
So how does that relate to a single player game like GTA? Ultimately what I'm saying here is that the player makes the game what they want it to be.