Thought it was pretty clear upon reading it that Misa knows Io was assaulted against her will, but just doesn't know/manage any other way to deal with it. Doesn't really matter if she genuinely considers it "cheating," the situation made her angry, from feeling powerless in particular I surmise, and this was simply the way she ended up reacted to it. To take her anger out on Io in the only form she knows.
On a sidenote, as comandeering and abhorrently awful Misa's reaction was, it doesn't feel "forced" in the same way it did from Hasa. She bosses her around, humiliates her, but she doesn't physically force her. Io still complies.
Like someone mentioned, I can imagine we get to see Misa feeling bad about it all afterwards, but also Io probably not being as angry about her reaction as she ought to.