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....which is to never, ever ask a question to which you don't want to know the answer. I don't think the boyfriend is crazy - I think he couldn't help himself and couldn't set his curiosity aside for the three seconds it would have taken for him to ask himself "Do I REALLY want to know this?". This takes maturity and self-knowledge, which the boyfriend clearly didn't have.
My own example: I had a wonderful boyfriend when I was 25 who I loved and who I know truly loved me. He asked how many partners I'd had and I had roughly four times the number he had (he'd slept with three women). He kept bringing it up and bringing it up, more in a passive aggressive way than anything else. Finally, I turned to him and said in a very serious tone: "I want you to stop bringing this up. If you don't stop bringing this up, I will break up with you. It will kill me because I love you, but I will do it" and said nothing more. It worked - he didn't bring up my sexual past ever again. It was a sticking point for him and he did the math and realized that foisting his insecurity on me wasn't worth losing the relationship.
We broke up for other reasons and I'm presently married to a wonderful man who has never once asked me a detail from my sexual past. We both obey the golden rule on that one!