sorry but isn't fmc idea he started making a move,from the 1st scenes with katrin and kevin.U forgot when the partent was visting,the apartement that mc and fmc bought.Plus he only heard about brian as katrin,very very later in the game.
Yes the MC did try to talk Katrin into choosing him instead of her lover onces he found out she had a lover.
Didn't really work though and didn't get him very far and yes that was before he found out who the Lover was.
But IT doesn't change the fact the FMC DID later came up with the Idea on her own.
Or that she then asks the MC if he would be ok with the
IDEA.
To get Katrin away from Brian and instead have her take the MC as a lover.
Or that she then decided it was better to have Katrin to join them both instead if she was into the idea.
So not sure what your point was.