OP, i really want you to put comments from this forum to one side for a minute, i know in my experience most of the comments i had tended to make things 10 times more hurtful than was actually helpful!
One of the best things someone did said to me though was that 'only you know the bones of your DP'. Are his actions worth separating over? People do make mistakes and by the sounds of it his best man had a lot to do with this. You sound like you are defending him against a lot of negative comments which i think answers the question in your own heart of how you truly feel. Is he a good husband, father, son? Do his good qualities outweigh this one off incident?
At our counseling session she said in some ways its the realisation that the image you always had of this person has changed. In some relationships this can happy early on, in others it happens further down the line and it can be potentially more hurtful in this way. I always placed my husband on this pedestal and part of this was that i never even questioned him after the stag do because i just assumed it wasn't something he would ever do. I know this is different as you did stipulate this before hand, but i'm guessing in a hummer its been a difficult situation to avoid, in comparison to actually walking into a strip club. I told my husband in the counselling that i couldnt get over it and didnt think i could forgive him for, the counseelor said that you should not be expected to just get over it but rather take your own time to work through your thoughts and feelings. It was also ok for me not to forgive him, its something that really hurt me and i shouldnt have to, it doesnt mean we cant move on though, does that make sense?
The issues i had following our lowest point were basically around my personal body issues, and these are still ongoing. after having DS i dont look like i used to, stretch marks and baby belly just make me feel so inadequate and i still have images of this gorgeous skinny tanned girl all over my DH. This sounds similar to what you're thinking about, but when ive spoken to my DH and i said you must have fancied her and wanted to have sex with her and obviously now im a disappointment, he said no, beauty is in the eye of the beholder. Its true i guess, I dont fancy all the guys at the gym and their buff bodies, and i didnt notice when my DH put on weight because i love him and have always found him to be completely sexy. Your DP wants to marry you, he's had a child with you and he had the decency to tell you what had happened and how much he regretted it.
I'm not saying its going to be easy, there are going to be highs and lows but in my experience i had to accept that my DH made a mistake and move on. When i see him with our son and when we spend time as a family i couldnt imagine it any other way, i never wanted to leave him, i was / still am hurting, doesnt mean i dont love him though.
I really do hope you sort things out and have an amazing day at the weekend, i'm so sorry you're going through this at a time when you should be so happy and excited. I would seriously remove the BM from his position though, it will only anger you on the day seeing him. Ask DP to make him an usher instead and ask someone else, its the least he can do in the situation. If you need anything else please feel free to PM me, i've been there so know exactly how you're feeling, even if its just for a rant without the fear of being judged on here!
Sending you all my best thoughts x