Is it weird that I don't want my partner to meet my fwb?

Hi folks! Wondering if you could help me understand this.

I've been with my partner for almost a year and had this fwb for a little longer. In this time, my partnership has turned into a serious relationship, while my fwb has remained an occasional outing or sleepover.

I really enjoy how my fwb has felt like a space separate from the rest of my life - somewhere I can go to get away and have some fun and connection. They're not involved in any of my social circles, as I met them through some folks I no longer engage with.

My partner, however, seems to find this strange. They seem to want to meet my fwb, having expressed interest many times and confusion around how I don't want all of us to hang out. I have met people my partner is involved with, and they date within our local community, while my fwb lives a little further away.

I like how this works for me, but my partner seems disappointed and maybe threatened or insecure about it. I've asserted that I like how it works, but they have continued to make comments about it that make me feel like they're not entirely comfortable with this.

Have any of you been in a similar spot? Looking for advice and perspectives. Thanks in advance!