I personally abhor hypocrisy. In your shoes, I'd be telling him "there's no way we're going to explore this possibility for you if we don't explore it for me also."
He doesn't own you so he cannot "share" you. You can share yourself. You choose to share yourself with your husband. It's your choice, and only yours, if you choose to share yourself with someone else. Did you have any choice when he shared himself with those other three women?
Now, that's all about me being defensive. So let's move onto your situation. You've been lurking here for months and you say you've never really felt monogamous. So is it fair to say you're pretty comfortable with the idea of polyamory?
For your husband, this is all new as of two days ago. He grew up in a society where monogamy is the expectation, and cheating is frowned upon but generally accepted as normal. What you're presenting to him is a huge mental shift. He'll need time to get used to the idea.
It would be great if he could read Opening Up
. It presents a lot of different forms of consensual nonmonogamy in a format that's accessible to regular people. If I remember correctly, I think one of the interviews is even a military couple not unlike yourselves.