By chaperone do you all mean some old unmarried Aunt knitting in the corner and the couple sitting several metres apart. Why not just say NO. If they respect you they will stop, if they dont respect you and try to carry on LEAVE its that simple.
God gave us freewill, there is no magic bullet or one straight jacket fits all approach to Chastity except saying NO and be aware of your humanity and your sexuality and what they are for. Thats what Catechism was for, the Churches teaching on sex these are enough or should be if you believe that you are the people you have been portraying in this thread.
I think Occams Razor is the solution here, dont make life more complicated than it needs to be. After a marriage must be based on trust and respect, if paramours can not be honest with each other and respectful of each other, and their wishes/conduct when they are alone, then how can they make a Sacred Oath before God?