Threesome

So, my boyfriend of over year signed us up for a threesome app. We’re young and into experimenting. He brought it up with me and I nodded along thinking it was just a fantasy of his. It is, he just actively wants to fulfill it. I’m flattered he wants me in it I guess and sure, I’m happy he fees comfortable enough to share these ideas with me. However, seeing him flip through pictures and bios of other girls made me stomach turn. I feel bad if I shut it down, I also can’t go through with it. I know I need to just talk to him. I guess I’m just reaching out. It feels as if I’m not enough for him sexually. Do you think he secretly just wants another woman?