The premise of this manga is so flimsy... "Sempai thought that possibly/maybe she saw a girl that looked like Mai (from a 5 years old photo) in a brothel... let's fuck our way through all of Tokyo brothels to check if she's been mistaken or not..."
Right... this is just a pretense so we can see the cute-shy MC have sex with a panel of various women.
Inb4 the sempai was actually interested in fucking the MC from the beginning.