IMHO, Sakura did tell Nagi and Mio about the cucboy too, so Nagi tried to soothe (comfort) her
Then Mio asked Sakura in chapter 134 "what did HE say to make you unhappy?"
So, she did tell Nagi and Mio about cucboy and she might tell them it's just a one-time sex, so Nagi linked it to her own...