Do you think Zoey did fall magically in love with him while being away? No, she just became aware of it. Thats the theme of the two, they were both too young/confused to be aware enough of their feelings.
And that's why I think Zoey would've been okay with it, because she wasn't aware of her true feelings and in her mind at the time they were simply fooling around.
You drew comparisions to Jill or Quinn in another post but these are different. In quinn's case its outright stated by her that shes not looking for a relationship and very clear about it.
Yes Quinn says so, but you can clearly see that she feels more for MC quite early on in the game after you had sex with her (or at least that's how I interpret the look in her eyes). To me it seems that Riona and Quinn are the same: They say they aren't interested in something serious, but deep inside the feel the need of wanting more when it comes to MC, with Quinn realizing it much earlier than Riona.