I think the best thing you can do is continue with therapy....ALONE.

Based solely upon her actions (denial, discounting, etc.) i have to say that your wife is acting in a totally self centered, immature manner. Telling you that that your feelings are "wrong", refusing to have sex because suddenly, after 16 years, your emotional state reminds her of her family and she no longer feels "safe".....no wonder you feel depressed and resentful.

Her actions are contradictory. She claims it was a drunken, one night stand, yet she goes out of her way to make this guy feel welcome, IN YOUR HOME. Something just doesn't add up.