@eyeswideshut,
I'm not sure what there is to 'handle'.
Let's call your girlfriend Jane and her girlfriend Sue (it just makes it easier).
Jane moved away from Sue and gave you the key to the new place. Apparently Sue and Jane have now reconciled. In the meantime, you are in a semi-open marriage where your husband apparently doesn't mind whatever is going on with you and Jane (or Sue or anyone else, for that matter).
All I can figure out from what you've written is that you're a bit jealous that Jane has Sue (at times) and so her attention is not fully devoted to you.
You need to let this go, and recognize that your husband's been awfully patient about all of this. Time to take a leaf from his book and be accepting of Jane and her relationship with Sue. Jane is not with you 24/7 and that's impossible, anyway, as you both have lives, plus you have a husband and family.
This could be a decent situation all around, as your husband would know where you were, Jane would have Sue when she couldn't be with you, and apparently Sue is okay with it as is your husband.
Use safer sex practices, as it's not outside the realm of possibility that Jane or Sue could occasionally take a male or female lover (HIV isn't the only STD out there), or your husband might as well. Keep your children mainly out of it as they don't need to be mixed up in this drama, although you should not lie to them. And maybe sit them down, with your husband, and explain that sexuality is a spectrum and it varies, and not all marriages are alike. But that a relationship composed of people who truly care about and love each other, and want each other to be happy -- that is the kind of relationship you hope they get into when they are old enough, regardless of the gender of the person(s) they are with, how many, etc.