Why does he want two girlfriends if he loves one of us more than the other?

I have been talking to this guy since June. The other night while I was staying the night at his house he decided to tell me about a “girlfriend”. I was confused because I thought I was his girlfriend despite us not talking consistently since June. We stopped communicating for a while but got right back in touch which was why I was even back in his bed but he didn’t decide to tell me about his “girlfriend” until now. He starts to go on and on about this girl he loves and doesn’t want to lose and how he wants us meet one another etc so at that point I knew she knew nothing about me. Some days later I overheard him call her on FaceTime while I was there and he tells her about me and even shows me to her in the camera and she was silent for a few and even hung up but he called right back and told her “I just want us to try this out. I don’t want to lose you and it wouldn’t hurt to try having two girlfriends. If it doesn’t work then it just doesn’t but I want you to know that no matter what no one can take away the place you have in my heart.” And that hurt to hear but I decided to give it try and with contemplation she did too. Fast forward to when the three of us actually hung out together and her and I finally met. The three us of stayed the night at his house together, nothing happened because either of us wasn’t quite ready for anything sexual with the three of us yet. The next morning I happened to be off but she had to head to work. She kissed him goodbye and gave me a hug and left and him and I had the remaining of the day together while she was gone but he seemed to be out of it and seemed to really be missing her even though I was there. I even saw he texted her “I put on your oversized t shirt you left because I miss you and started thinking about you as soon as you left.” I couldn’t help but feel jealous because i’ve grown to actually like him more. I don’t get why he wants or needs two girlfriends when he clearly loves her.
Why does he want two girlfriends if he loves one of us more than the other?
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