If not Okada, who else?
Of course it had to be her.
Well that is something new I haven't seen before: A teacher paying a student to have sex.
I think it was "natural".
Okada is that sort of guy, and Maki is that sort of girl.
Makes me think, if maybe her decision to sell her body is in part influenced by her friendship with Yukari. Maybe Maki's family is poor, and being friends and spending so much time on Yukari's house made her with for money. Maybe her hypothetical poverty is a reason why granny gives money to her.
So, Mari wished for money, not to ear money, but to have it.
How could a girl like her on a small island get that money? That are few options available, one of them was Okuda. And Okuda, as bad as he is, probably isn't the type to force himself on anyone, instead he just looks for girls and woman that are willing and Maki was one of them. It's sort of a business relationship, each one gets what they want.
The real test to how bad Okuka is, IMO, is if he will discover that Sakura was posing nude to Gin and use this information to blackmail her into having a sexual relationship with him.
I think in the end it will all be resolved by Kuroki's action when he starts to see what's going on.