How to trust again

My husband told me one morning last week that he and one of his co workers had planned 24 hours in advance to have sex and that the next night after they got off work he followed her to an empty parking lot and that they had sex in the back of her car. He says it only happened that one time but I have a real hard time believing him because they work at the same job and see each other at work. He says that they dont talk anymore at all but I dont believe that because everyone who works together talks. I cant believe this even happened in the first place. The night it happend he sat home all day with me and acted as if everything was fine even tho in his mind he new what he was gonna be doing that night after work. He had plenty of time to know that what he was about to do was wrong on so many levels but no he didnt, he went ahead and did what they were planning on doing and now I am left feeling sad, hurt, disrespected, disappointed and not know where to go from here. Right now I dont feel sexually attracted to my husband and I definitely dont trust him at all and dont know if I ever will again. I am lost and confused and dont know what to do. He says he wants to be with me and that I am the only one for him and that that one night was nothing but a mistake and it will never happen again but how am I to believe that when he had those 24 hours to think about all that and never did. Now I have to decide weather I stay in this marriage or I go my seperate way alone.