A friend.
If you are gonna discuss s3x are you more comfortable talking with a friend or spouse? Or maybe a professional, a stranger or no one at all?
Spouse
Spouse, almost exclusively, unless I need medical advice.
Spouse or physician.
When I first read your question, I thought, what is 3 X's? Lol! A best friend.
Friends, spouse, spouses of friends and friends of spouses, OK. Been there, done that.
Strangers..., hmmm, maybe metaphorically or rhetorically.
Professional?...professional what?