The fact that there is a pannel of her in lingerie doesn't mean they had sex. She's had very similar moments with Shinba too, and nothing happened.I've read chapter 29 and it seems I was almost right on the money about the guy she is hanging out with
The guy was her first boyfriend, and presumably but most likely had sex, with her lingerie showing off at his home, adding with Hizumi saying things like "You don't have to dream or work as long as you're alive, I'll do anything for you. So please don't leave" like wanting for him not to change and just stay this way. Then, she ultimately ends up getting cheated on by some girl during Christmas, and pretty much ended the phone call with a "Farewell, Hizumi". The ex boyfriend says that he's very fed up with Hizumi spoiling him but to him is more of an annoyance. The butterfly flew while the caterpillar walks off and all that. And then we get back to the present of her current relationship, where she swore to never let Shinba change with her hugging in the end of the chapter. And like I said again, this may not be the last time we get to see the ex boyfriend after the mom arc, so we may get to see more of their past. But yeah, the ex boyfriend's a dick for cheating on Hizumi.
Ha ha...
She already hinted to Shinba that she's a virgin when they were at the hotel.