BB
You are so right about the anger direction bit. Like you I got involved with someone I did not know to be attached. I actually got suspicious and tried to get out, but was persuaded not to with a pack of clever lies. When myself and the other half found out about each other it was suddenly all my fault-all the blame was laid at my door.
AND yes men do often let significant others find out, sometimes even telling them themselves then come over all helpless and pathetic.
If they chose to be with you it's their choice, however for self-preservation you do not need to be dealing with any of this. I'd be straight and tell the guy you can see him, but it's not your job to deal with his family.