Is it normal for my boyfriend to ask me if we can do a threesome

My boyfriend of a year texted me one morning. In the middle of our conversation, he says "how would you feel about a threesome". i simply told him that it seems awkward. I asked him why he was asking and what brought it up and he told me that it was just something he has always wanted to do. I get turned on at the idea of a threesome and porn videos about them but i dont know if i should actually go through with it. I feel uncomfortable with doing it because it sorta feels like a way of him cheating on me or something. kinda like a lack of respect. please help. i dont know what to do!