If he hadn’t married her it would be different. Once he married her, her children were his. Societies have an interest in making it hard for a man ro leave a woman with children even if she is unfaithful. This particular aspect is the bedrock concept of marriage and why marriage exists.
Female infidelity is a ‘bedrock’ of society and marriage. . .to allow females to be a tramp is why we have marriage (monogamy anyone?).
If you say so. . .but not in my world.