Thoughts:

-Kind of the big issue with not talking about your sexual history is having your sexual history spring up on your partner out of the blue. I imagine had the two of you talked about this asshole before hand fiance would have been able to handle the confrontation at the party a whole lot better. Pro-tip: Your past is your past and it won't ever change, it's better to find someone that loves you regardless of your past than to find someone that loves you as long as you're able to hide your past.

-FWB is legit a piece of shit. Like straight up scum. Don't really have much more to add to this but god damn, what a shithead.

-The initial dishonesty regarding sex with this dude is prob gonna be the longer lasting issue. You're telling your fiance sex with him is better because you love him, but you just got done lying to him about faking orgasms. Can he really trust that you enjoy sex with him, or will he believe you're still just lying to protect him? Dishonesty is cancerous in relationships.

-Fiance is immature, and unfortunately that immaturity is possibly gonna give FWB what he wants. If he were here, I would tell him he needs to work very hard on learning to accept that a persons past does not fully dictate who they are now or will be in the future.

-Fiance is immature, but resist the urge to hold it against him. Yes, all those feelings of jealousy, insecurity, and emasculation are all very immature, but they're also very valid and built on a very real confrontation that should never have happened the way that it did.

-Give fiance space, tell him something to the effect of "I love you and whenever you're willing to talk I'll be here," but right now his emotions are almost certainly too highly charged to carry on a healthy, productive conversation about things. If he never does come around, well, that's life. Fortunately, ending a 14 month relationship def isn't the end of the world.