bringing someone into the mix??

I had mentioned to my boyfriend of a year and a half and I had mentioned to him about trying a three sum. At first he thought I was trying to trick him but eventually he said he wanted to try it for his birth day. I know it was something he really wanted to try (although he's only mentioned it once through the course of the relationship. Now as I said, it was something I had mentioned and there obviously would be some boundaries and would only be a one time thing... I've been reading forums on peoples opinions of it, however; I can't help but be weary that it will change our relationship. So the point is, I would like to know you're opinions on why you think my boyfriend (who I live with) or any man for that matter would be alright with a three sum in a committed loving relationship: because although it is something I'm considering for him, I can't get past that part of me takes it personal and questions his intentions.