Boyfriend wants a threesome. 🙄🙄🤦🏽‍♀️🤦🏽‍♀️

I was really confused today when my boyfriend told me he had a dream so I asked him to open up and tell me about it and he told me he had a dream that there was some girl on girl action with me and another girl and his fantasy has always been to watch that happen. I’m a quite open minded person so when I had heard this I was quite weirded out at first but then I asked him would this mean you having any physical / sexual contact with the other girl and he said no I would just have sex with you and wouldn’t feel comfortable touching another girl yet alone having sex. I would just want the other girl to have sexual action with you. Now I know all the taboosand negativity related to threesomes and a relationship but he would not have any contact with the other girl so I wouldn’t feel jealousy but I am just worried because I’ve never done something as crazy as this nor do I even know how to be or act with the other gender... also I’m only 21 and he’s 22.