While I do find it strange, I do find it hilarious that both the top voted comment and others seem to be so judgemental. Or to quote:

I'd bet that he probably has other weird "quirks" /issues

This is the only "red flag" I've seen from this guy. I think he's awesome and I love him but

Yep, she's given so many hints that there's most definitely more to it than just this one thing.

But I will give the guy one thing:

He confessed to the reasoning behind his question. When she showed that she didn't understand and was not happy, he then showed her exactly what he was talking about. He discussed it with her and she again refused.

He then stopped bringing it up. It's she that turns around and brings it up again - asking to permanently destroy it. He says he is understanding to her beliefs, but that it's something he'd like to keep.

So far - other than it being pretty freaking weird, he's done absolutely everything right. Shy of them both sitting down and discussing an alternative. Maybe ask him to put it somewhere out of sight out of mind, like in storage or a bank vault IDK.

People have all sorts of weird creepy things, and while this is a step above the rest, I'm actually half-impressed with how well he presented it. Especially considering how quickly people would immediately break up over it.

Whatever - If there's absolutely no other dealbreakrs, if there's no contact between these exes, if he's not bringing it up over and over again, then ... I can't fault it. If it makes you uncomfortable, you voice it like you have, and he voices his concerns like he has.

Honestly it's just a question of is this a dealbreaker to you, or would you be able to let one dark skeleton live in the closet.