Careful OP, seems the board is recommending the end of marriages to the top poster's this week.
You love him, he`s sexy, you enjoy the sex together, just wish you had a bit more of it. You are not at fault, neither is he. It happens. It can be a confusing thing when your husband/lover is GOOD in bed, but tends to get distracted with other priorities.
People are not perfect in all ways, and I think you know that. I think you understand the logisitics well of any outside relationship. To be quite frank, I think you can probably eventually have what you want, but it will take time, and transition.
Your husband wants to keep a 'handle' on the situation somehow. People do this with all sorts of things. Sometimes it is kissing, sometimes it is intercourse, sometimes it is anal sex. Hell, I know people who had a rule about not making a specific meal for the outside party, as it was 'their' dish.
Every situation is different. As you suggested, it might be to protect your outside 'image' or whatever the case may be. There are a lot of people who screw up with NRE. Innocent parties end up paying for it. He is considering unchartered territory, and wants to have a tight rein currently.
What you COULD say to your husband, is you will adhere to his wants, for a period of time. If you prove discreet and things go smoothly, you expect changes, period. You will not be on a tight leash forever. You will make sure not to forget him, the children, or your responsibilities.
Prove this, and then talk again. Its a good way to keep a marriage intact, yet work towards your needs.