For example let's say I have a Luxray with Rivalry and I hit a Genderless Pokémon like Magnezone. Would it:
A) Get a 25% boost B) Decrease a move's power by 25% C) Nothing happens
Rivalry:
Deals more damage to a Pokémon of same gender
Genderless and male/female are not the same gender.
Source, if Rivalry would have triggered it would have mentioned it under Luxray's HP bar.
C nothing happens there is no boost or decrease
Source used Pokemon with rivalry before
Nothing would happen. It's like trying to attract asexuals. It doesn't work very well.
