He got tempted. The situation might have been very tempting and he wasn't getting as much sex or excitement from sex, and he lacked self-control. Even committed Christians can fall into that trap sometimes.
Well there are alot of reasons if he isn't happy at home alot of married couple get to Comfortable in there marriage and stop working at it sometimes its just easier to stay married then to get Divorced sometimes they stop having sex actually there are tons of reasons so I guess you would have to look into their marriage and figure it out
your either not having sex frequently. or maybe it is not good ot maybe your not as sexual as he is. my ex use to want to have sex daily. instead we only did about 4 times a week. he cheated to i had to go. and my first boyfriend was a virgin and was horrible. so i was about to cheat but instead i left. i have dated both gender so im somewhat use to what each likes to some degree. and guys need it. good and offend