I just wanted to say for me personally it was a little bit of a shock that you stayed with him. Just the way you described it for me was a major red flag for "cheater". Hearing that he was that way he entire mature life made it sound a lot worse.
As for staying with him. If you want to open up the relationship for him (and not yourself which I couldn't do, just my opinion), I would suggest honesty. Be honest with him. Tell him you would like to open your relationship up.
I personally find that purely sexual relationships are a myth for long term sex relationships. Feelings develop over time in my experiences (I've been burned before).
Opening up the relationship is something you have to decide for yourself. You seem to understand it pretty well. There might be some advantages for you to do that but I would still stress that you make absolute sure and talk it over with your husband that this is definitely what you want.
After you figure that out, test the waters. I personally do not find dadt(don't ask, don't tell) relationships very easy to work so I would personally have to have my partner inform me about at least some minor details and above all else, I would HAVE to meet his sexual partners myself.
Now that could cause jealousy if you're not used to it. I'm not advising you that you have to. I'm just stating that in my personal opinion, I would HAVE to know the other women. Even if it was just basic little things of chit chat at a coffee shop.
I hope you guys have a good open, honest discussion and I hope your relationship remains strong.
Female - 24 - poly