Why would a woman want to indulge in infidelity? It gives a woman no more children - if she wants more children then she can have them by her mate (assuming she is married). The reason is that, given that she can have and raise relatively few children, she should choose the best mate that she possibly can for those children. Unfortunately there are a great deal of women who would also like the best male available, so she may not get him, but may have to settle for a less attractive man, so that she has a partner to help raise her children. Imagine the benefit in having a partner to raise the children, but to actually have children by a more attractive male.
Hence infidelity is rational for a woman, but only if the lover is more attractive than her partner. Of course her husband shouldn't know about this, as there is not biological point in raising someone else's children, but if the partner is blissfully unaware of this, and thinks that the children are his, then the mechanism succeeds.
To summarize, women are interested in quality.
The boxes below are here to give advice for the many, many hits this site gets from people hitting the search engines about male and female infidelity:
She has been unfaithful because deep down she would prefer to be with the man she had her affair with. Biologically, if she felt that you would give her the best offspring, there would be no reason to have sex with another man. In her eyes the other man is more genetically attractive to her. She might try to explain her behavior by saying that you are more sensitive, or understanding, or whatever, but in reality she is only with you because you are the most attractive man who she thinks she can get to stay with her and support her.
Your relationship is a loser. Every day you remain with this woman you will be reminded of your inadequacy. Whether she is attractive enough to make this worthwhile is what will decide your future.
Firstly don't tell him. If you can possibly get away with not mentioning it, do so. If you can possibly get away with lying (saying that you were both too drunk, or you decided at the last minute that you couldn't go through with it etc) then do it. People are very good at believing what they want to hear, and this will be easier to lie about than you expect.
If you feel the need to confess, then convert to Catholicism and tell the local priest or something. Don't assume that just because you would be willing to forgive him a similar transgression that he will do likewise. Male and female infidelity are fundamentally different.
If you have told him, then your relationship is probably over. There is no genetic benefit in raising someone else's children, and you will have just cast considerable doubt over the paternity of past or future children. Even if there are actually no children, it doesn't matter - it's in the genetic programming. Even if you can demonstrate paternity though a DNA test, it doesn't matter - it's in the genetic programming.
But if you want to try to save the relationship then do some damage control. It is critical that say that you did it because the other guy showed he cared for you (or something similar). Don't make the mistake of saying that the sex was good, or that your lover had attributes that you partner doesn't, and talk about the sex as little as possible, and as dismissively as possible. If you want your relationship to survive, then you partner must think that this was a temporary thing, and that your partner's shortcoming were only behavioral.
Use cliches like 'I was lonely' or 'I just needed to feel loved'. They are your best bet.
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