I imagine that's something we'll learn in the future. From the little snippets of dialog in the nightmare, it seems like it was more than one person, a gang maybe. It could simply be a matter of being in the wrong place at the wrong time. Or, if it's a gang, maybe the MC accidentally offended the wrong person, and the rape and abduction of MC's gf was retaliation. There's a few possibilities. Let's just let it play out.
These harem games are not usually the games I go for. But there's enough intrigue in here that I'm looking forward to seeing how it plays out, especially with this Zoey character.