Since in season 2 we saw that the siren's song can't affect Rupert because he isn't into women, here's some theories on why Flora could affect him in season 1:
The plant monster's seductive powers work differently from the siren's, they're more powerful and don't depend on wether or not the victim is actually attracted to the monster
The mushrooms messed with Rupert's head, either made him believe he was actually attracted to Flora or just countered his gayness
At that point of the story Rupert was just way too repressed for his gayness to cancel out Flora's influence
If y'all have other theories I'm all ears 👀














