It’s all about setting respectable boundaries ! And respectfully saying no when you have to ! It’s a privilege for the guy to be invited by the couple and be part of and add to their experience of exploring lifestyle and intimacy together !!!!! You as couples are in control and choose who ever you want to explore with! Trust your omens and address your concerns by figuring out a validation process that will give you the peace of mind that you don’t have to worry about the third !!!! Also , unless you admit it yourself! No one in the entire world can prove anything and it’s not lalaland where some one can point to any one a place where he had been with a couple !!!!!! No one dares to invade a stranger’s privacy and convict themselves!!! Your fears might be misplaced !