3 Way vs NO WAY!!

My husband wants a 3 way. He says it’s something he would like to take off his ‘bucket list.’ I told him him he should’ve checked that one off before he got married to me! I don’t want him to cheat and at least he’s being honest and open with me. He’s dropped the subject but I’m still wary. I honestly don’t know how to feel about this. I’m all for sexual exploration, but a 3 way for me defies the sanctity of our marriage and I believe can lead to all types of issues. Yet, I feel bad that he confided in me and I shut him down like that. I also feel aggravated like, ‘ Dude, I’m not enough for you?’ Please help me sort out these feelings!!