CSN, you say DP thought you were "perfect".. well he's at fault too.. no-one can live up to that.. nobody ever is perfect! Say to him, 'well sorry.. if it's perfection you want, you will have to leave and try and find someone who can live up to that.. but if it's someone who loves you, who until now, you loved as well, warts and all, then maybe you should reconsider." (Not that I am trying to put words into your mouth or anything.. 
)
Also point out to him that many strong people put relationships back together whereby one party has actually outright cheated.. and this is not the case here, not anywhere near it. The only problem (large problem though it is) lies in the fact that your friendship was with a man and therefore you unwisely kept it a secret because you feared what he (DP) would make of it. You know realise that was a mistake because it automatically makes your 'dealings' with this man, innocent though they were, seem underhand and like an affair, but in fact nothing could be further from the truth. So if he ends this relationship it will not be for the reason he thinks he is ending it (ie you having been unfaithful).
Tell him there isn't much you can do about him seeing you differently now.. but perhaps he is better off looking at both you and your relationship together without rose-tinted specs. Perhaps you could suggest that much as you love and him and want to stay together with him, that he is also not perfect due to his behaviour when he has been drinking and how it makes you feel (but that you do love him warts and all!) Suggest a fresh start.
I don't see what else you can do really. I kind of think that you were maybe entertaining the idea (if only ever in your head and with no intention of acting on it) of 'something more' than just friendship with this other guy, but that now the poop has hit the fan it has driven all thoughts of that right from your mind and made you see you were just 'playing'.. and hey, we've all done that (well many of us!).. because nobody is perfect and we all feeling flattered by the attention of the opposite sex, and will be more susceptible to it at some times more than others.. and mostly our our private thoughts are our own to entertain ourselves with!.. but you don't have to share these thoughts with DP (!)... just maintain your innocence and persevere.
I hope it works out ok. It's still early days. Keep the lines of communication between the two of you open as best you can, have no contact with the other guy and tell DP exactly what you have said to him about this and why, and DP may start to see the light a little.
SJ x