Need help - no judgment

My boyfriend of 5 years wants to have a threesome. I’ve juggled the idea and I am not full opposed but something keeps holding me back. Not sure if it’s the fear of catching something, but I’m also feeling some insecurities too. He’s done nothing to make me feel insecure, just some stuff I’m holding on to from before him. But my question is for those that have had a threesome, how’d you get over any opposing feelings you had to do it? Or do you have any advice?