AITA if my wife (26F) wants a threesome with another guy and I (28M) don’t know if I want to?

Didn’t know where to share this so I’m trying it here. My wife (26F) and I (28M) have been married for a couple of years and have been together for about four years in total. We get along pretty great and have always clicked. Sexually we have always had great chemistry I think. She has expressed to me in the past that she has a fetish of being shared with another male while I join in and we will role play but leave it up to fantasy for the most part. Somewhat recently she expressed to me that she gets off with being shared and even having explicit or suggestive pictures shared with others. She tells me that it comes from wanting to be admired, and that even though I’m enough it’s a fetish that she just has. I’ve indulged in it a bit myself and found to enjoy the fantasy side of it and have even enjoyed sharing some of her pictures and what not with others, but again it’s just an idea.

Now even more recently, she’s floated the question of being shared in an actual threesome with another man. I was a little taken back by this because again in the past it’s been more of an intangible connection between this fetish and reality. I also don’t think I’m “well endowed” as I range from only 4-5 inches, and worry that may be part of the potential need for someone else. I’m not sure what I should do, I feel like she wouldn’t mind if I flat out said no and left it at that but I also don’t want to disappoint her if she has other needs. I’m just not sure what to say and I don’t want to disappoint her. What should I do in this situation?