A lot of things could be the reason. 1. Impotent: A man's inability to satisfy his wife in bed could make his wife had sex with other men outside.
2. Finance: Some women are materialistic, They cheat on their partner if he is not financially buoyant. Most women passing through a lot of hardship are mostly tempted to have sexual intercourse with other men than their husband in order to pay for their bills.
3. Foolishness: Some women are selfish and foolish in nature. Even when their partner is sexually and financially OK, they will cheat.