Threesomes

My boyfriend and I have been dating for 4 years. For the past 2 years around his birthday he has asked me for a 3sum. In the back of my head I'm like nope, not going to happen. I don't want another chick in my bed trying to out due me and my man. We have had our issues with him still messing with his ex girlfriend and he asked me to do it with her and him. My answer is still no and for him to ask me to have one with her almost kills me. That pretty much ended our relationship but even though we were able to make it work and get back together, why do I feel like if every time he asks for one it's like he is cheating on me with another female in my bed and my face.