You should explain that he is not obligated until you exchange vows. Insist on releasing him and part for some time, so that you are sure he is not marrying you under pressure. That is to say, give his ring back and if you have a date set, cancel it. You two don’t have to give a reason to anyone else, save that you reconsidered and you aren’t ready to marry each other yet. OTOH, you have no reason to expect that anyone will keep any secrets, except by mutual agreement.
This act of integrity will clear things up so you can lay a new foundation. You do not want to attempt to build a marriage on the foundation you have, which is one on which he doesn’t trust as being sound. If you get back together, re-build the relationship on a new foundation, one based on total honesty. If there are going to be things you keep private, make sure you both agree on what may be kept private, what may not be kept private, and what means of keeping tabs on each other are mutually acceptable.
Oh, and next time, realize that it is OK to say, “I’m not ready to talk about my sexual past yet” or “There are things in that area of my past that I would find intensely embarrassing to admit. I can answer any questions you’d like about temporal consequences that might come in the present or the future, but I’d really rather forget it ever happened”…or the like. In other words, honesty can’t mean that no detail has been left unsaid. You don’t have time for that.
He’s going to be one with you to deal with any future fall-out from that past, so he has the right to decide what he needs to know about you past, if he wants to. You may mutually agree to spare each other the gory details of your past sins, if you want, but it must absolutely be in the open that this privacy agreement was made, and that it was made not under duress, but because you both agree it would be better not to disclose.
Nobody is here to stone you, though. Go, and sin no more. We’ve all been there, but by the grace and mercy of God, we hope to always do better thereafter.