Educating people on the fence can work in your favour .. I just don't know about the wording...
"I came across on the web" sounds way to flighty.
You need to come down hard and certain that this affair is destructive behaviour. I am certain this guy can find something else on the web that says affairs are perfectly ok...
"I want you to understand how marital therapists deal with infidelity - they don't ignore it - they reccomend speaking up just as if the partner is drinking too much or gambling their life savings away... this paper is from a professional who deals with this every day - and I can find many more from other family therapists that will tell you the same thing"
That sounds more confident than "I found this on the web"
You can close with "I respect your right to an opinion, but I have a marriage to protect and its my responsability to ensure everyone's opinion is an INFORMED ONE."
I would like to discuss this further with you if you would like to know how you can help... I have left my number on the article, I won't pressure you further if you don't call it.
