Wife’s Best Friend

Met my wife twenty years ago and got married three years later. From the start I met her best friend and over the years we’ve socialised and know each other well.
About three months ago, drunk, we kissed and then carried on. Since then we have sex weekly and it is good. Passionate and incredibly adventurous with both of us very open in telling each other what feels good and what we’d like to try such as her asking me to take her anal virginity and me saying I would if she would let me cum inside her when I did.
We have agreed to continue but with no desire to break up marriages.