Oh gosh, I am so, so sorry to read this, CS.
I'm with the crew who are VERY surprised that it took him 6 months to come out with what I would call VERY important information. I could never, ever, ever, ever ever ever (you get my point?) be with someone who believed this.
Ugh. Who knows. My ex claimed he could never and would never cheat on a person. We all know how that turned out, if I'm on this site. So someone who is more open about it - I could see it either being much worse (he's basically telling you he will cheat on you) or, possibly? better (he's being open about what he might do, and possibly? will be open in the future?) Who cares, really - I personally couldn't take the risk.
I was also wondering how a person who knew your past could say such a thing. In that light it would be downright cruel. But, knowing that he DIDN'T know, that is slightly different.
I don't know - I get keeping the past the past, but it's also a part of who we are. I am not sure what I will do if/when I'm dating again, but I think I want potential partners to know, eventually, where I'm coming from.
My heart goes out to you! I'm so sorry. Good luck navigating this, it is so hard.