You know, I originally bring up this event as a cautionary tale since I found the idea of discarding humanity
(and breaking Reimu and Marisa heart) to be depressing, but somehow, everyone seem to actively try to trigger it instead.
Oh well...
This events chain can starts once she's at love status, but requirement at love stage are much higher than lover (more than 20,000 favorability). Final stage before event are triggered require more than 90 jealousy points which increase by 4 each time you encounter her during the dates, increase by 2 or 4 points when having sex and getting discovered (depend on conditions) and increase by 10 points if you didn't meet up with her for too long.
(this one even come with specific lines when she 'explode')
Also, remember that this wasn't a stand alone event. This event were tied in with Udonge tree progress.
Jealousy point doesn't matter if Udonge events wasn't triggered off-screen before.
You can also track progress of the events chain by tracking her character profile.
That being said, I never have seen anyone trigger this events chain naturallly before so it might not even working properly.