Threesome??

My bf's fantasy is to have a threesome but I'm kinda self conscious about the outcome. Does he not find me attractive anymore? Idk I'm very open to women. I dated a couple girls in high school and had occasional sex with one of the girls I dated. I moved on to having sex with guys and after that I didn't consider girls anymore. It's hard. I'm not as confident as I use to be. What should I do? Will this ruin my relationship?