Thank you so much for this. It's impossible to find a complete translated version of the webcomic so you doing this is amazing. And I'm so curious who were both of their ex's?
I read the whole webcomic and I can tell you about their former relationship history and virginity status but spoiler alert:
Hori confirmed that she never had a relationship before miyamura and in this chapter it's also stated that hadn't lost her virginity to someone before miyamura and also never having anything with sengoku
So she's practically in her first ever relationship/sex
While miyamura did lose his virginity before but it's not stated when he did or to wich girl but its also never stated that miyamura had a girlfriend before and the way he's thinking and being with hori also implies that this is his first relationship
I assume it was ,,heat in the moment" thing like he said about his tattos and piercings
Hope this explains it for you