On the issue of "Invasion of Privacy"......
That would seem a two-edge sword. I agree, privacy is necessary. But how do most people find out about their mates having lovers, other wives, other children? Do the offending mates just blurt it out to them? Perhaps sometimes.
But usually there are clues, clues lead to looking around from the mate not sharing sex/lives with others. How does the mate do this looking around? If they ask the mate, and the mate says no - then what? Just say ok and stick her/his head back in the sand? Isn't this akin to what the elders are being damned for in child molester cases? Asking the person "did you do this?" Molester - no. Elder, ok - we respect your privacy and your word. Don't want to pry into another person's life. End of story. After all, usually there is no hard proof.
I think some common sense is called for. I have no answer for DreamMaster except a real life experience. A friend of mine was engaged to a man who was romantically involved with a woman on the web. She knew he had been talking to her, but became suspicious when it was so frequent. Tthen the engaged woman confronted him. He absolutely denied everything. She then invaded his privacy - and found all the receipts for roses, then dinner, then jewelry. Again, she confronted - he acknowledged his web romance gone real time.
What was she supposed to do when he denied it? Say ok, let's still get married? Stick her head in the sand and say there's nothing there? That's the way women & men (particularily women) have done for centuries, and I don't feel it's for the good of a relationship.
Somewhere, there's a middle road - but putting instincts aside and head in the sand, butt in the air to be kicked is not middle road, imho.
waiting
ps - I put my head in the sand and did not invade my first husband's privacy for his first 4 girlfriends and continuous selling of drugs. Then I took my blinders off - and had witnesses against him for divorce within a week. Not only did I invade the bastard's privacy - I took him for every damned penny I could get for me & our 3 kids.