Welcome to our forum.
You and your husband should probably have a long talk in which you decide and express what each of you can compromise on, and what each of you can't compromise on. Then cross check that with things each of you absolutely want/need in the marriage, ane what each of you can't live with/without.
A lot may depend on how important this polyamorous stuff is to your husband, how far he really wants it to go, how much of it can he live without, how much can you live with. To me his actions seem to hint that he wants to go further, but you have to decide if there's any way that can be acceptable. Perhaps if he goes quite slow, and gives you a chance to get to know the new person (whoever that is)?
I wish there was a way to guarantee your marriage will come out intact. That depends on many things. Even if you don't divorce, there's a chance you could end up with a marriage where he resents you or you resent him or both (and that's not a very good option either).
GalaGirl's post has a lot of good info in it, so read it through closely a couple of times.
Glad you could join us.
Love means never having to say, "Put down that meat cleaver!"