Emma being open minded does not equate to her being okay with Ian actively inserting himself between Wade and Cindy, and taking advantage of the situation. Nothing about Emma so far seems to indicate she'll be cool with it. She might just end up being willing to talk to Ian about why he would do something like that, unlike Perry or Wade who would likely refuse to speak with Ian outright.Wade and Perry are hardly a loss.
It's not clear how Emma would feel about it, since she is open-minded (though not an imbecile). Alison doesn't seem to be a close part of the gang. Jeremy is already on Ians side.
Jeremy isn't exactly on Ian's side either. Chapter 12 seems to hint that Jeremy doesn't really approve of what Ian is doing with Cindy, but is at least willing to keep it a secret on his behalf for now. He did let it slip to Lena.