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We've all done things that have caused us feelings of guilt, shame, or, in some cases, just experimental curiosity. When we're in a committed relationship, we don't have to reveal everything we've done, but some information becomes relevant to our partners so that they can put things in perspective and understand where the other person is coming from.
As someone who's in her 50's, I'm rather surprised when meeting someone in the same age range who hasn't "done some stuff" (or claims not to have). The point really is whether it's stuff one has moved past, should have moved past, and/or incorporated into one's present-day values. For example, if I dabbled in something that was considered not exactly legal thirty years ago but have long since given up, it doesn't make me a hypocrite to be blatantly anti-illicit substance with my children. It's a part of my history, and I may choose to reveal that to an adult partner, who, having lived through the same era, could probably be able to relate to it.
The issue, of course, is acceptance by one's partner. The LW's BF could have had a reaction something like, "Wow, what a mindblower. I never figured. But that's in her past, and it's something that brought her to cherishing a one-to-one committed relationship, especially with me." But there's something about the act itself that he cannot come to terms with.
While that's sad, it's also an indication that there may be OTHER things in the LW's past that he would not be able to accept--who knows? If his image of her is one of not having a history, not having experimented at all, not curious about life, and not having developed her philosophy and values from such experiences, then maybe he can only deal with a person who's a tabula rasa--and, surprise, surprise, no such person exists.
The LW should overcome her sadness (understandable though it is) and move on to find someone who's more accepting. She need not reveal specific experiences, but feel that her partner would not reject her if such a thing were revealed, spontaneously, intentionally, or not. There's no point in trying to drag someone through therapy and so on if he's simply incapable of accepting a certain history. It doesn't really make him nuts; it's just a compendium of his own experiences and values that are not compatible with hers. She shouldn't spend more time with someone who tries to make her feel bad about herself and a history that she can't change, even if she wanted to--which, of course, she may not.
LW should move on, appreciate herself for who she is and what she's become, and find someone who can feel about her likewise. Don't try to wring love and acceptance out of someone who simply can't give them.