If a solid family unit is in the interest of males, why then do so many males indulge in infidelity? This has on obvious answer if the male has no mate - impregnating a woman clearly has advantages whether or not the male is involved in their upbringing. The more women a man sleeps with, the more children he is likely to sire.
We can see here the origins of the Coolidge effect. We find a strong inclination for males to be unfaithful to long term partners, and this is basically for the same reason. Even if a male is in a relationship, there is tremendous advantage in impregnating other females - the children are more likely to die than the ones that he raises with his wife, but the investment (a few minutes of time, and a few million sperm-cells) is so small, that he may as well make the investment. Even if the partner is relatively unattractive, and unlikely to even raise a child, the expected benefits still outweigh the costs.
In summary, males are interested in quantity.
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:
This is going to hurt, but it's not actually the end of the world. Just because he had sex with her doesn't mean that he prefers her to you, or that he will leave her for you. On the other hand it may, but remember that biologically he saw an opportunity to (perhaps) replicate with almost no effort. Biologically, to produce another child, with only the investment of single night's energy is a pretty good deal. Even if the mother were quite unattractive, and the resultant child probably quite unattractive, it's still a genetically attractive investment.
As for the future of your relationship, the best measure is how much investment he has in the affair. If it was a drunken one-night-stand, then it is unlikely to have a serious effect on the marriage. However, if it was a prolonged affair involving presents, money and commitment, be very concerned.
On the other hand, in this day of DNA paternity testing, a clumsy mistake can be very expensive, and you may end up sharing his income with the mother of any illegitimate children. Assess this risk when you decide on the future.
Firstly don't tell her. 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, 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.
But if you can't get away with lying about it, or you have already told her, then do appropriate damage control, and you can probably save your relationship.
It is critical that say that you did it only for the sex. You must play down any prospect of a long term commitment which will take anything away from the marriage, or any children in that marriage. Talk a lot about being controlled by your hormones (that will be very believable), talk about the sex if necessary, but don't talk about emotions or feelings for your lover.
The best phrases to save you are the old cliches 'it didn't mean anything' and 'she meant nothing to me'. These became cliches for a reason - because experienced adulterers discovered they worked.
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