You can get each girl to like inflations via one of two methods.
The mini-pump, which you can put onto characters. You do need a certain likability to put it on them (believe it is 20?), and once they put it on once, they get a small interest in inflation. You can immediately take it off, then have them watch you inflate yourself, and each time you do, their interest will grow by 3. (right now 40 is required for characters to inflate from the shower hose)
Second method is the gas pill. After giving one to a girl, skip a bit of time (20-30 minutes), and their belly will start growing. Once it does, a short interaction will occur, which will increase the inflation interest by 1. That's enough to get the interest ball rolling as presented above.
(I do think that them inflating themselves should increase their interest once they have interest, but it doesn't.)