Hi there, Composer and welcome!
I must say -- even though jealousy and possessiveness and all that stuff are feelings we consider generally negative, it is refreshing to read about a guy who admits to being jealous of his girlfriend wanting to be with a woman. All too often, we see the man in a relationship discounting another woman as not really being an actual person with whom their partner would potentially like to be with, and instead they turn it into this non-threatening inconsequential nothing, or a titillating element that he can get his rocks off to, as if the women are only getting together for his satisfaction. That gets so tiresome to read again and again! So I applaud you for acknowledging this development in your relationship in the way that you are -- it shows respect for what women can have together.
That being said, it does sound like you two are "doing it right" and taking care of each other as you explore this possibility. Gathering information, going slowly, talking, examining your feelings and belief systems. Keep reading the boards here and ask questions as they come up. Glad you're here!