HELP, 19F married to 21M

I 19F have been with 21M for 3 years and married to him for 2. I don’t know who to talk to about this, seeing as i don’t want my/his family to see him different and we both are apart of the same friend group. Anyway, the last time my husband and I made love, he asked “would you be willing to have a threesome” in the middle of doing it. It caught me completely off guard, and kinda made my heart ache. When we first got together we had that conversation and we both agreed that it wasn’t for us as it seemed wrong. The next day all i could think about was that question, wondering who he was thinking about that wasn’t in the room, and just going down the rabbit hole. We later talked about it, which led to him saying “I do wonder what it’s like to have sex with someone other than you, but i only want to do it if you’re there to. if that makes sense” and tears came to my eyes. For background, I am the first woman he has made love to, he had a crush on me since the 8th grade and after pursuing me till i had no where left to run we got together senior year. I however was in a not so good relationship prior to my husband, but it is still an experience i had with another man. I guess what I’m asking is how do i handle this in a more mature way? Is it bad that i feel this hurt? What would you do if your husband told you he wanted a threesome to experience another girl after only being together for 3 years? I don’t know what to do or how to react to this…