It is not about honesty with you, it's about his own history. I can imagine how many times he wanted to talk about it but couldn't. You keep it in for years and years and no matter how much you love someone you are afraid to share. It's mostly a guy thing, society forces you to be strong no matter what... This happened to me and not just once, someone whom I loved told me to share some of my history. I did, and they reacted (instinctively) badly. I realized no matter how much I love someone, some things I will keep to myself, even If I get a feeling that sharing would help. I guess it's the same for him.
As for therapy, forcing him to open up or else I think is the wrong approach, maybe he doesn't want to, maybe he's awkward about it. Chances are he wanted to die without sharing this with anyone...you forcing him will(maybe) make him resent you...Contrary to popular belief here, a lot of men find talking about problems counterproductive.
And what he is doing isn't wrong, "wrong" doesn't exist, 50 years ago being gay was wrong, 100 years ago being a different religion got you killed in some places...you get the point.
What if he had a different weird fetish ? Lets say he liked shit/piss stuff. There are plenty of videos for that on the internet( and jsut about any other fetish out there). Would you think that's also wrong ?
He has a condition, his brain works a different way. If he never showed any inclination of hurting you or doing something bad, than I don't see anything wrong in that. IF ( and that's the whole point ) he was able to deal with this all his life, without you even knowing or worrying about it ( was he hurting you during sex?) than I thin what I said is the best way of doing things...
At least that's what i'm thinking, as someone who can empathize both with him and with you( been in similar situations). You being confrontational about this in any way is the worst thing you could do, even forcing him to open up. Just tell him therapy is up to him and you won't judge either way.
This is just an opinion though,a controversial one according to this sub, so take it as you will.