Hey sav, you won't automatically get a penalty for turning down a girl's plan if you click "no" and text them a reason. Depending on their personality, they may still get a negative texting memory associated with it, but that's expected behavior.
You can also attempt to start a scene with both girls to appease them. For example, if you have a scheduled plan with Sarah to go to the Gala, and Mary asks you to hang out, you can use the free text prompt to say "Go to the Gala with Mary and Sarah". It might be a little immersion breaking though.