Threesome

My boyfriend wants to have a threesome but I'm not sure if I want to. The idea of it sounds nice and I've always told myself I'd try it at least once. Let me also mention that he told me only if I was comfortable doing it because he doesn't care either way. And he also said it didn't matter if it was with another guy or girl, it is my choice. I know I'd snap seeing him with another girl so I know that's out of the question. I asked if he would really be okay seeing another guy fuck me and he told me as long as I'm okay with it. I just feel so weird about it. How are you okay with seeing someone fuck your girl? Is it still a threesome if there is only penetration between us and the third party just gets eating out/sucked? I feel like since I'm overthinking it so much maybe I should just not do it at all. What do you guys think?