Don't do it!
Although I am not in the US military, I have over 20 years in the Canadian military and getting divorced is going to leave you high and dry if something happens to him.
Right now, you are entitled to some benefits if your husband passes away or God forbid gets killed in the line of duty. If he divorces you and remarries, you will be out in the cold and entitled to nothing.
I agree with others who have already commented on him being a serial monogamist. I think most people are naturally serial monogamists in fact.
"He's already acknowledged that he will let me go to be with her"
Let me be very suspicious for a second. If you all get together and plan an amicable divorce between you and him then no doubt they will convince you that you don't need to press on things like support or pension sharing "because you'll all be together anyways".
If you get a divorce, and I think moving on is something you should be thinking about, then make sure you get a lawyer to look out for your interests. You need the full protection that any person getting a divorce would get...don't be lulled into signing agreements under the belief that you will always be together. You're going to lose out in this unless you prepare to actually be divorced and independent.
Look after yourself my friend...this is a scary and preposterous idea in my opinion.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over