I am so sorry to hear you are suffering this awful trauma...for that's what it is.It's not just a one night stand it's the ripples that go out in your relationship, your dh in doing this has totally "moved the goalposts" on how you feel about him and your peace of mind and you are probably thinking "how could he, not my dh , why? what have I done etc how could I get him so wrong?"
I know how you feel as my dh nearly 3 years ago now had an affair with a work colleague which went on a few months. Our situations are different in that this is a one night act of stupidity- my dh had afew months when he "lost the plot" and almost lost everything he holds dear.They are the same however in the devistation you feel and the issue of tust.
We are still together and stronger now...all I can say is firts, you do not have to forgive him- some things are unforgivable- but you do have to find a way to make peace with this-or it will eat away at you and your relationship. The only way to try to do that is good honest no feelings spared talking with him and he has to do the same in return. He must tell you everything!!!!
I think it's very important to check your health aspect- if he didn't wear a condom you need honey to get checked out and so does he.
Next I ask why did he tell you? It certainly wasn't to make you feel better- it was to off load his guilt and make him feel better. He has demomstrated complete selfishness in his actions in doing this and in how he's told you.You need to make him realise that he has risked everything and this cannot be forgotten overnight. it is now his job to work hard on putting things right and appreciating you and his family...the trust will only begin to come back if he goes out of his way to reassure you and keep you informed of all his movements.
On the other hand do try to put this in context- if you can belive what he's saying and it was 1 stupid night- weigh this up against what you've ha d with him to date. We do all make mistakes in life you can recover from this IF YOU WANT TO- you are in control- he's risked everything. It's not so much the sex issue- though that is bad enough- the real problem now is his selfishness and the trust and you now feeling you don't know this man whom you thought you knew so well.He has to put that right....All I can advise is to talk talk talk and set aside time for each other every week , make sure you date each other and this cannot be a taboo subject. He will want to forget about it- he will probably accuse you of not moving on when you raise it again and again but it's likewhen you've been say in a horrific accident- the trauma of it makes you replay over and over what happened until you can make sense of it- then only then does it stop being traumatic and you can move on. It takes time believe me but it can be done. Please do not give up...if you think you have a relationship worth fighting for then do that...it hurts but it can bring you closer. take care.