I think cake eating would fall under something like, he was still having sex with the OW but expects to get to have sex with you too? Along those lines anyways for an example.
The concept is to not let him have everything he wants from both relationships. That your boundries would be, you cant have both, cause im moving on to bigger and better. Without it coming off as an ultimatum of you asked for a separation, this is what you get!!! I'm not here for your emotional plan B cause your plan A had a bad day. Take care of you emotional needs, without being treated like a doormat cause your only interested in saving the marriage. You'll gain no respect from him or yourself. But at the same time, address your issues within the marriage by listening and validating what got you there, but that your changing into a new person that would be worth getting to know.
I could be wrong, I am many O times. But that's the gest of it. Decide what is too much for you, only you know, and when the boundry gets broken, you have to enforce the treatment/disrespect. For example, don't take his calls/texts for a week, make him email in advance for options with the kids.
