Any 14 year old boy who wasn't raised by wolves knows it's wrong to sleep with another man's wife.
If my son slept with a married woman I'd blame him plenty, since that isn't how he was raised.
I completely understand this, and for society to function and to keep from getting killed, your expectations of your son are spot on, but the moral responsiblilty lies with the married person. The single person has made no promises or committments.