If he weren’t married and had been paying child support (I’m against this btw) then there would be an injustice. Because he married her, there isn’t.
Has he been wronged by her infidelity, yes.
Is that cause for divorce? yes.
Does that annul the marriage? no.
I find it too common that people think marriage is about a guy having exclusive sex. Marriage is a societal institution set up to make it hard for a man to leave a woman with children. Marriage is about children and the needs of children.
You could not find 10 men on the planet to agree with that nonsense.
And NONE of them are active on Free Republic.