polys can be jealous.
I got involved with a guy whose girlfriend wanted a polyamorous lifestyle. She swears up and down that she is biologically polyamorous and can't just be with one person. I was the one who went into the relationship (with him, her boyfriend) with a monogamous mindset "giving polyamory a chance." In the end, she experienced more jealousy than I did. In fact, I wanted to hang out with her and be friends with her, but she couldn't deal with it and has essentially forbidden him to see me.
I think it's kind of backwards.
I don't think of it as jealousy so much as insecurity. She must think I'm going to displace her or something. I don't know. All I know is that I can handle knowing she's around. I think very highly of myself and think whatever I bring to the table for him, she can't provide. And whatever she brings to the table for him, I can't provide. No need to be jealous or worry about being displaced.