I have a few questions.
You said earlier in the thread that if your husband had come home and said he wants a girlfriend, you would not have agreed to that. So, I guess the fact that he said he already fell in love with her and wanted to protect and teach her was a major part of your willingness to accept and welcome her as part of your household and his lover.
Now that this is tabled, do you think your acceptance of him being with her has opened up new possibilities for polyamory in your relationship? Are you going to look at it as more of an option now, and perhaps open up your relationship? You never know - he might come home two months from now and say he fell in love with another woman, so I'm just wondering if you might change your perspective on if he asks "for simply a girlfriend," as you put it. This whole situation could be one stage in a new development for you two.