I think it is unfair to write off a male just because he doesn't feel comfortable with the past promiscuity of his girlfriend. Yes, it is a weakness, not a strength. But that doesn't make such a feeling less real. Is there really no room for irrational male sensitivities?

That said, the way it is described here does seem unhealthy. If he knows he has problems dealing with it, why did the two of you think it to be a good idea to disclose so many details? That only triggers his fantasy, his insecurity, and makes it part of the present and not of the past, where it belongs.

So I do think this is a major issue in this relationship. But there is nothing you can really do. This is his problem and he needs to learn to deal with it. If this gets into a dynamic where he asks for more details, and you decide to lie to make sure he isn't hurt, and that leads to him being more insecure, since you are lying about it, that leads to a vicious cycle.