I agreed to a open relationship just to try something new, but why am I feeling jealous?

I started talking to this guy back in April. We started to become close. I would stay the night at his house. We would go downtown to ride bikes, electric scooters. He made me feel like I was the only girl he was talking to and that he actually loved/liked me until I started to notice him always asking for a threesome or mentioning having another girlfriend or how whenever he got a text when I was around he’ll say things like “It’s not who you think it is.” When I didn’t even say anything so it made him seem and sound suspicious and there would be times where I would tell him I’m coming over but he would give weird excuses as to why I couldn’t as if he had another girl over. After he continued to mentioned having two girlfriends one day I just told him “I’m not into girls even though I will say a girl is pretty if she really is, I guess trying a three way relationship wouldn’t hurt. It’s something to experience and it doesn’t hurt to try.” Right away he says “I’m glad you mentioned that because I’ve been talking to this girl. I really love her like I love you and I want you guys to meet each other.” It ended up happening. After we both saw pictures of one another and after he mentioned we were both sort of alike. The three of us spent about three days with one another and there was no drama. A little awkward at first but it went better than expected. The only thing is he seems to love her/like her more. Now that he isn’t hiding anything when he texts her he’ll show me and say things like “Girlfriend number one is on her way.” Meaning I was two, that had made me jealous and then he showed messages of him telling her right after she had left us one morning to head to work “I miss you and started to think about you So I put on your oversized t shirt that you left over here.” he says things and does things that makes it seem as if he cares about her a lot more even though he calls himself having two girlfriends.
I agreed to a open relationship just to try something new, but why am I feeling jealous?
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