My bet is that she's lying to some extent, considering he says in chapter 15 that he didn't release anything because of the perfect-dry-shot control. Also considering how he would have released an gem if he did, as he didn't release one earlier, she's probably trying to manuiplate him into forcefully entering a relationship with her, which she brings up.
Hell, I'd even say that he never had actual sex with her. You want to know why I think this?
One, she ran out of the room with a beet-red face, meaning she was embrassed and eager to get out of the situation.
Two, if you look carefully, in the video she was sent, both Miyoshi and MC had their underwear on, meaning it's more like dry-humping. She wouldn't be able to get pregnant from that.
Three, MC thought that he was with Ayame during that vision, who was pregnant with his child. It's unlikely he would have vaginal sex with a girl who has a child he wants to be delievered so far into her term. Hallucinations aside, it's unlikely he would have gone that far to begin with.
Four, earlier it's said that if a witch brings their partner to orgasm, then the other witches will leave them alone and consider their prey "taken." Meaning, if she DID manage to get him off, the rest of the witches still wouldn't be after him.
So, yeah, she's lying her ass off to ruin his relationship with Ayame and try to force him into a relationship and get him out of school.
But, if she's not lying, we all know how this ends...
@Siquall