you have been friends ,with both of them for a long time , you used to be lovers with the girl in question, do you not think afterwards the friendship will break down .... it was ok with them doing that stuff with strangers but it might not be a very good idea when he sees you shagging his wife he night just flip out knowing you were together before they were i hope you see my point don't think i have worded it correctly .....don't think with your cock on this occasion!! be careful , hope you make the right decision