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We vow to love our spouses in the wedding ceremony. That doesn't necessarily mean sex. Love and sex are different. And it certainly didn't mean to be available at the other's orders.
If a man struggles with staying faithful to his wife, particularly while his wife is carrying his child, that means he doesn't WANT to be faithful. He's a poor poor example of a man, of a human. And a woman who cheats is a poor example as well.
If you don't want to be faithful, why get married to start with? Or once it's a struggle, once you don't want to be faithful, why not end the marriage so that both people can be with people who respect and love them enough to be faithful and to want to be faithful.