I have not viewed the LS post. I may make some points about it from what people are already saying.
But my opinions here at this point is about what the OP has said (I will note when I am referencing LS).
Dear OP. That seems to have happened years ago and that she didn't tell you out of shame. Now most guys do NOT understand women completely, we never will. But we do need to talk to them and understand their actions sometimes. Hell, my wife has done that. Done things she DID NOT want to do, to make me happy - but in reality, I never would have done or do things she doesn't want to. So she was hurting, but didn't want to "lose me" or make me upset.
I think she met you, after a few dates and liking you, falling in love - knowing HOW you are so black & white on sex - that she couldn't tell you. As you have stated, that sexual number and doing gangbangs were beyond your comfort level.
Think about this. Your path with this women was due to her PAST. Everything that happened before - lead you to her TODAY.
I think you are being hard on her, and if she has been honest and faithful otherwise - then I think YOU are in the wrong. You had no control of her past.
About the xBF: all of the sudden *HE* springs this on you? He may be dumb - but he gotcha. He planned this when you and her got engaged. People do this kind of crap ALL THE TIME. They want something for themselves and will hurt others. Hell, a crazy woman told my wife lied about ME - which help break us apart last spring. crazy woman also wanted sex with my wife and other strange crap. (LS: seems like he should be in jail)
Either you need to stop seeing the kid (and therefore this woman) AND say you are sorry and won't see her again OR you need to APOLOGIZE and go to couples counseling with her. Sorry, it sucks that the xBF showed you video of sex acts. But view it as watching a crime. Don't be such a typical political jerk and punish the victim. (referring to women last year that were raped and yet their rapists get off with a slap on the wrists)
If the LS story is true (Again, I don't know it) or that was her past and she is done with it out of shame - and that she feel in love with you and has been faithful to you all this time. Then you are likely throwing away a good thing. You are punishing her for something that was in the past and that you were both played with by the xBF... who wants to HURT you, and destroy her. You are HURTING her big time. Guys like that will tell her "See, you are trash. you only deserve scum like me."
You need to be the better person, because right now - she is far more hurt than you are - and you are in control of the pain of both of you.
Get therapy for the both of you. Don't break someone's heart for stupid reasons.