>>279804301
There was a familiar aspect to it, both of them were victims of abuse and so they both were unconsciously bonding over their lack of unconditional love. But it was inevitable their relationship was turning romantic and would have turned sexual because Satou couldn't help herself as this was the only thing she knew as she was sexually abused by her aunt as a kid and as she grew older she only understood sex as necessity for any type of relationship. This is the reason why adult Shio only remembers Satou as her love partner and not as her family, their relationship already had escalated to that point.