@Raknasuu tbh, I don't even think Chika and Sakiko are having sex. I think she just said that to Minato to shock her, cause that's just how Chika rolls (also, she said "I don't know what else to call it", meaning that it's not literally "fuck buddies"). She wants to seem like more of a badass than she is (she certainly looks the part, so it helps), cause in the past we've seen that Sakiko is the actual top in the relationship. But they are already dating pretty much, they are shown to constantly hang out, act like a couple, they kiss, they say they love each other aloud, what else do you need? They just don't wanna admit it, cause, again, they're afraid to (two soft-hearted badasses).
And I'm gonna go on a limb and make some more predictions: Chika and Minato are gonna start being very buddy buddy with each other in the future and it's even gonna turn into a bit of a joke with Hibiki, Sakiko and Ema looking at each other and going "what the hell happened? When did these two get so friendly?" and then Chika and Minato just laughing about it.