Threesome

My boyfriend keeps asking me to do a threesome and I really don't want to, he told me last night that eventually he'll get tired of asking and maybe leave and not because I didn't do it but because he will feel like I'm not all the way in the relationship. I told him I would do it because I love him but he keeps asking me when like I can just make it happen, and he doesn't want to duck the other girl he just want me to eat the girl out and I'm not comfortable with that but I said I'll do it. I guess I just want some advice on threesomes if I should do it or not, and would you do it with your man?