your being to hard on your self.

If you don't want to do it then that's fine. plus you shouldn't have threesomes anyway. that's just a bit slutty. also you will regret doing a threesome more then regretting not doing it.

tell him that it is a bit messed up to do threesomes. that's why we only marry one person. not two other people or more because usually you just have one perfect love. not more then one. which proves that the other person that he wants to do the threesome with is more important to him then you or you are more important to him. try change his perspective so he realizes what he wants before he makes the mistake of doing it. because once somethings done, its not undo able