Am I insecure in my relationship because I wouldnt want to have a with another woman?

I think it would be fun to engage in a fantasy of my boyfriend's but I dont want him feeling pleasure from another woman. I feel like everytime we had after I would be competing with someone. It might ruin for me. Would this maybe be a more appropriate fantasy to fulfill with a husband rather than a boyfriend? Knowing that your husband wouldnt leave you. I dont know marriage isnt even a constant in life anymore to many so maybe even then it would be uncomfortable. I want to fulfill his fantasy but I think I would be jealous and upset. Maybe with someone neither one of us knew? I'm a pretty confident girl but he and myself wouldnt want to have a with an ugly girl so she would definately be hotter than me. Do you think this means Im insecure in the relationship? or is this a natural reaction to this kind of proposition?