Trust and Parenting: why men are less likely to forgive adultery

Several of my female friends, under the influence of a few drinks, have argued that they would find it easier to forgive their boyfriend or husband for having sex with a random stranger than for having a close emotional friendship-relationship with another woman. I pointed out that this argument would still require, you know, actually forgiving the guy for sleeping with another woman. I can't imagine any reason to forgive a woman for cheating on me, and yet my female friends admit that they would forgive a man for just about anything.

I think the opinions above are representative of men and women in general: Men are less likely to forgive a woman's adultery than women are to forgive a man's. I wanted to find something biological, something instinctive, that could contribute to the difference. I think this is it:

A man needs to know that the child he is raising is actually his offspring.

When a couple has a child, there's no question who the mother is. The identity of the father is only in question to whatever extent the woman is not trustworthy. If a man knows that his wife has been unfaithful to him, he may wonder whether he is really the father of his wife's child. A woman faces no such uncertainty; if she gives birth to a child, she's the mother.

I know that a man can be a great father to children that are not his, but doing so is not proactively Darwinian -- it does nothing to pass along the man's genes. For a number of reasons, the genes that led a man to have his own children were more successful than the genes that led a man to help raise others' children instead of having his own. If a man is going to be monogamous, then, it wouldn't surprise me if he has an instinctive need to know that he is the only person his wife is going let knock her up.