In my opinion, at some point it will be revealed something like, (MAYBE and ONLY my thoughts, NO insider info!)...
...Hiroko had an ongoing sexual relationship with at least her dad just as Matsu did, that pre-existed her relationship w Matsu. Due to that is why she was depressed, and either because she'd found out she was pregnant or didn't realize until they were dating - and that kept going perhaps against her wishes even after she & Matsu "went steady". Not sure if existing game timing supports that as I'm going from memory so don't bother roasting me if it doesn't. But yeah, something like that would neatly wrap it up, to my thinking.
As for the whole cheating thing, it's been my long-standing observation that many Japanese titles in the porn realm (which I'm pretty sure ES is inspired from given, well, almost everything about it) seem to have a strong double-standard for male cheating vs female, similar to that older western chauvinist view that men should and women shouldn't, but still common in today's Japan behind closed doors. Now that's my observation, and I'm neither Japanese nor a scholar of their culture (dilettante student at best). Really just manga in general, but I can't read Kanji so who knows what happened before the translator touched it... lol.
