Update! Boyfriend wants a threesome

Edit: We had a lonnnggg discussion about it last night. He did apologize for making me feel pressured into it. We have talked about it previously when we first started dating a long time ago so he assumed that I was still interested in the idea. I expressed how I felt about the matter and he said that he will never bring it up again unless he is joking about it. He also agreed that bringing someone else into our sex life would probably cause damage so he is willing to let go of the fantasy. Our relationship feels a lot better now after talking about everything. Thank you all ladies!

For me, it's just a fantasy. Do I like watching it in porn? Sometimes. But doing it in real life with my boyfriend... Probably not. He would want another girl. A couple nights ago he brought it up and I was uncomfortable because I guess I thought he was my "forever" person. I tried to tell him that maybe it was just a fantasy of his but he said it was a "bucket list" thing. I feel like he sees me as disposal and I'm scared he might break up with me if I don't do it. I said I would think about it if it was a person that we did not know. As if we would NEVER see them again. He agreed but later on I still feel uncomfortable about it. I never fully agreed.

I want to tell him that I don't want to do it, but I am unsure as to how? I'm thinking of telling him something like "I really don't want to do it. If you aren't willing to remove it from your "bucket list" then you should find another girlfriend." I don't want to be mean about it. I am unsure if he will ever get over this fantasy.