It was during the episode Love POkemon STyle, during the johto league compition, This girl named Macey was falling down a ditch and Ash caught her and she fell in love with him. THen it went something like this:
Macey(really close in Ash's face) : If I win Ash, you have to take me on a date!
MIsty then gets between them and shoves Ash out of the way
MIsty: sorry but Ash is booked!
Macey: What are you his girlfriend?
MIsty(blushing and stuttering): WH-what? No, I-I'm his coack!
Macey; HIs coach huh? Well then this isn't any of your buisness right Ash-
(Ash is running away at this point)
Does that help Max?