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Let's not worry about who is "nuts." We all have different standards for sexual behaviour. In my personal world, a threesome is really rather mundane, not near the edge of what I would consider relatively normal at all. For some people virginity might be an issue. The real issue is not the letter writer's sexual history. That's the trigger here, but it isn't the most important issue.
The big issue here is that the LW's boyfriend is treating her badly. For some reason, he is using this as a weapon to emotionally traumatize the LW. If he won't go to couples counselling, the LW has no reasonable choice other than to leave.
It's going to hurt, be sad, a huge loss. However, sticking around in a relationship that has become emotionally abusive and where there is no reasonable hope that it is going to get better is a mistake. It sounds like the relationship has crossed into that emotionally abusive territory.
If the LW's BF would engage in couples counselling then perhaps there might be some hope. As most couples counsellors know, by the time a couple walks in the door the prospects for the relationship aren't great. The LW should be aware that even if couples counselling were a possibility, it probably wouldn't be a quick 5-session fix.
To the LW: Nobody has the right to consistently treat you badly and shame you. As I read your letter, I'm angry on your behalf.