Was Satoshi sleeping with the girl that was supposedly a "friend."? i never really understood if he was sleeping with her or just pretending to make Naoyuki jealous.

Also, on Volume 4 we never find out if he actually was cheating or not, but the way he was acting led me to think that he was...

Satoshi never slept with the female friend he kissed in the car. At the very end of the 2nd volume, he explains to Naoyuki that she's just a friend and kissing your female friends is normal.

I'm guessing that he uses this same reasoning for kissing his female customers in the alley. It's his way of keeping them happy.

While I don't agree with his nonchalant way of kissing. It sounds like he never had sex with any of these women.