My husband wants to add another girl into the relationship. What do I do?

My (32f) and husband (32m) have been together 14years and married 8 years. (I had relationships before him but he hasn?t.) A few months ago during sex we were looking at porn videos and came across threesomes. (I?m bi and my husband is straight). We started having dirty talk and I mentioned us having a threesome with another woman as it was going off the porn video we watched. We just described what we would do if another girl was there and i thought it was just talk in the heat of the moment and it was never brought up afterwards.
Tonight he said he wanted to speak to me about how I felt about adding another girl in the mix just sexual and nothing else. I asked him what he was suggesting and he said he wanted to see me with another girl however he didn?t want to do anything with the girl (only me) he said he just wants to be involved and watch me with another girl and just touch me not her.
I?m not sure how I feel about it to be honest. It?s not something I?ve considered doing as fantasy is one thing and real life is another and I?m satisfied with him no one else.
Now my mind is running 100 X a minute thinking I?m not enough for him and what if he prefers someone else other than me or has he already got someone in mind.
I?ve literally just gone quiet and not sure what to say or if I say no will he resent me.
TL;DR, my husband hasn?t been with anyone before me yet I?ve had previous relationships before him.


Source.