To be honest, I have never had a threesome with my partner. I have experience with threesomes+ when not in a relationship. The best advice I can give is not to try to change someone’s core values. I make no judgments about couples who do threesomes, but if it is more important than to you than the boyfriend, then cut him loose and you will both be better off for it. Sex with my partner is a very intimate and important part of my life. I would never try to convince him to do something he isn’t comfortable with nor would I even desire someone else in our bed. You both may have different views of sex and what feels comfortable.