Be aware even if this was the first time it was shown, storywise it is canon that this was a situation that happened before between those two with the encouragement of the third party present.
Saying the shown act took place earlier, like even before MC and the girl met would only absolve MC himself in the most literal way. Thing is, this is a game. Even tho the player (ideally) identifys with the MC, they are not the exact same.
The fact that it was "made to
ABSOLUTELY illicit a response from players, just as it was intended to" shown graphic and in possible anticipation to focus negative emotions of the player towards other chars makes it clear the dev was aware of that.
If i meet a girl tomorrow on the street im aware she most likely won't be Maria Theresa, a virgin without backstory who just waited for me.
Just as MC knows the characters in his world have their own story.
That doesn't apply exactly the same in games tho.
It depends on the correlation of space and time.
In the real world for me, and for MC in his world, there is a distance regarding time (it happened "long" time ago) and room (it didn't happen right here but in another place/city whatever) so u also gain emotional distance.
In a game, there may be such distance for MC, but the player sees it all in the same place (his screen) and at the same time (a few hours of playtime wouldn't really qualify as foretime yet).
Thats why the dev can wake/create a reaction in us, since he knows we r still "in that situation" no matter if its a flashback a time leap forward or even totally out of MCs perception.