Search Results: how to catch a cheater

We all know about the controversial topic of Cheating, im focusing on one aspect of it, being caught in the act. Why is it that when someone is supposed to be in a "committed" (i put that in quotes for a reason) is caught getting intimate with another person, the outsider is usually the one attacked?
Example, a woman catches her guy getting freaky with another woman and she attacks the woman instead of the person who is actually doing the cheating. Poor lady probably didn't even know that the man was with someone, now she is embarrassed. Same thing on the other side of the gender pool, the man attacks the other man rather than his promiscuous SO. See i can understand if it were someone who KNEW you were in a relationship and still decided to get dirty with your SO, then thats just grimy, but to attack a poor clueless stranger? makes no sense to me.
What really gets my goat is that after embarrassing the clueless stranger, the "victim" turns around to attack the cheater. Why not just do that from the start?
If the intimacy was going on in a residence, i can understand telling the stranger to leave first before rounding on the cheater, the act of attacking is the part im calling useless.
So can someone please explain to me why people do this? If you caught your SO in the act (with a stranger), would you?