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Or at the very least, a perv defender.
I don't know if the guy is nuts, it may be that he is just young and inexperienced, as others have pointed out, and so has no context, no life to fall back on, to deal with his insecurity. or maybe he really is deeply bothered by it. The LW should get out, keep the cats. Hell, it's too soon to be even be living together, for cripesakes. What's up with you young-uns, you seem in such a hurry to couple up?
Some element of curiosity about a partner's past is natural, I think, but when someone starts asking for details and numbers, that's usually a bad sign. On the part of the person asking.