I think your thread is a really interesting one.
You are coming across as very level headed and fair. Jealousy isn't rational - just because you already have a husband, doesn't mean that you aren't going to feel insecure.
For example, I have never felt jealous about my girlfriend's husband, because he already existed before me. I never felt too jealous about her other existing lovers. But new ones? Yes, I feel threatened initially.
Your boyfriend may feel the same way. He may worry that he is going to be replaced, since, as you say, you do not yet have the same level of trust and history that you do with your husband.
It is valid and understandable that these feelings may be the same for you, also. How can you assure yourself that your boyfriend won't find a 'replacement' for you? Or that you will remain equally as important to him, if he is involved with others? Well, you can't really. But you can both put your faith in each other's best interest, if you work at it.
My girlfriend and I had a lot of trouble in our early days because of assumptions. Assumptions even crop up now. So, we adopted the phrase "assumptions make an ass out of U and I".
In order to overcome assumptions, you have to take action to understand the true intent or meaning of something. It's also incredibly useful to talk about expectations and intentions, too.
So, let's work through what you said a little bit and break it down:
A.) Temporarily discontinued my external relationships for his benefit
Did you tell him that you were going to do this / that you had started doing it?
If so, did he agree to it?
Sometimes we take matters into our own hands and do what we think the other person needs. What you did was lovely - and very considerate. However, the problem with self-sacrifice is that it can cause trouble in relationships if not handled extremely carefully.
It might be that your boyfriend is starting to date others because he thinks that's healthier. It might be his way of encouraging more freedom - his way of reminding himself that he can't be monogamous to you, nor you to him. Best to ask him.
B.) No "right" to keep him to myself if he wants to explore external relationships, though I want to
Well, that depends on the structure of your relationship. I am not sure whether you believe in hierarchical poly. I'm not sure if your boyfriend could ever be seen as 'equal' in your V?
If you can only offer him casual meetings and he wants more from a longterm relationship, then yes, it wouldn't really be fair to keep him to yourself. If you still want other lovers and don't want him to have any, that's also not very 'fair', of course.
A closed V might be fair, if everyone wants it. Or a limited-open V, where you agree on how many extra people you can each date at one time.
C.) Been threatened by the thought of another girl being romantically involved with him
Again, here, it would be wonderful if you could talk about what the two of you want. Perhaps some realistic questions might help - for example, asking him what he wants from others he is dating and how he sees you in his future?
It doesn't matter if you haven't been together for very long, or even how serious the two of you are - expectations and guidelines help, even if they are very, very basic. A simple agreement on dating others, to be reviewed in 3 months, should help greatly.