The way I understood it is that she was already hypersexual (compulsive sexual disorder) before the events of the story. She can't turn off her horny and she has been fighting her attraction to MC long before his powers manifested. There are several times that it's hinted that control is a big deal for Parker, which probably has something to do with her hypersexuality. So the "pheromone" effect is a real problem because now it's making it impossible for her to maintain control of herself, which has always been a struggle for her to begin with. She's had her ways of coping with it, but it isn't working anymore now with his power changing.
As far as Riley, she's almost certainly affected by the pheromone too, but not in the same way as Parker is. Parker is easily the most vulnerable to it.