I don't know what happens next, but right now Sacchan looks like real bitch. After Ruki's confession without giving her answer she fucks with Remi and doesn't even hide it. Moreover she does this after her ex cheated on her!
So if she wants sex without relationships then just turn down Ruki and fuck all you want, but seems like she likes Ruki, so why she went to Remi in the first place? Well, similar shit happens in real life, but this particular case seems a bit forced.