What, why would he sleep with someone that represents a person that drives him into panic, ie his mother? What an odd comment, seriously. Sangwoo stumbles on the woman and her son and is visibly shocked, as anyone would be, but I think also he might be having flashbacks to seeing his actual mum in such an injured way too. So much concerning his parents deaths hasn't been divulged yet, so I guess we can only speculate, but Sangwoo, or the boy as you call him, is a wreck.
For me, when I looked at the woman, I thought she resembled Bum (look wise); Sangwoo has an obsession with his mother and even had Bum take on typical chores she used to do around the house. I saw it as she resembled Bum, so that’s where I was going. I didn’t actually know where it was going, but that was my theory... sorry if it was wrong. We are all entitled to speculate and come to our own conclusions. (Plus I haven’t been keeping up with this too often so I don’t quite remember the previous chapters all too well. Whoops)
Ooooooommmmmgggggg if that boy sleeps with her and hurts Bum.... I will raise hell!!!!