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The BF sounds like an emotionally manipulative ass and the LW should thank her lucky stars that this brought that side of him to light before she was even more committed to him and their entanglement even harder to escape. He feels threatened by her past, and feels the need to drag her down to his level by "picking" at her. Jerk!
If it wasn't this revelation, then it would have been something else eventually. Maybe she gets a promotion and a raise and he suddenly makes less than her and his ego is bruised. Or perhaps she shines to brightly in front of his friends some evening out and he feels jealous of the attention she is stealing from him. Whatever. This sort of behavior doesn't just start from nothing - it was there just waiting for the right moment to show itself.
Contrary to popular belief, a relationship does not require lots of hard work to make things work. Maybe down the road - after years together and several kids and new jobs and new responsibilities and the all of the dynamics in the relationship have changed - then adjustments might need to be made. But don't trade what should be an effortless and enjoyable time together for the half-assed comfort of not making waves in a less than ideal relationship with someone who has demonstrated that your past has more to do with his "here-and-now" than your "here-and now" has to do with your present togetherness.
I too, had a past, and out of fear for what she may have heard through the grapevine, confessed my transgressions to my wife early on in our relationship. I thought it was going to be a big deal - prepared for the worst - she simply shrugged and said, "eh. Everyone has a past."
That is the proper and adult response. Don't settle for less, allowing the BF his hurt feelings just to save a dying togetherness - Therapy at this point?!? - Forget it - it shouldn't be that much work. Suck it up and dump the schmuck. You deserve better. Demand it! Yes, breakups are messy. But in the long run you will be happier moving on if he won't.