I agree with others that looking at this third person as some kind of sex "pet" was probably not good, or or maybe you used wrong terminology. However, I totally disagree with these statements that there is no turning back. That is a bunch of nonsense.
Most of the people on here were at one time mono with their partners and decided to try and then keep being poly. There is a tendancy of some to think that because one dabbles in poly ONE TIME that there is only one way to go, forward, and the partner who wants to put the brakes on is never correct in asking for that.
Your husband started out dishonestly in this, and he is basically just slept with this other woman a few times and to think that it is necessary for you and he to split because he is now dedicated to ploy for life is ludicrous. Partners survive affairs as reconcile all the time so to think that if you two communicate that he will never accept going back to your previous arrangement is silly.
Of course you cannot "force" anyone to do something, but as your husbands wife you do have the right to strongly tell him you expect this experiment to stop and if he loves you like I assume he does, he needs to at least put a hold on this until you have a lot more private discussion with him.
The NRE will eventually cease and then you two can decide if you want to re-evaluate this dabbling in non-monogamy.
You are his wife, she at this point is NOT a toy, but she is just a person he has had sex with a few times. hat is NOT equal in any terms as far as i am concerned.
Going back to the original relationship does not condemn the two of you to a lifetime of misery. There are millions who do it. They just do not normal;ly post here.
Good luck to you.