I’m a woman, but NO. I’m really anal about this sort of thing. I don’t believe telling the truth in every circumstance is warranted, but in a case like this, definitely.
Basically you’re fucking with two people’s lives here. Not just the mom, but the baby, and the baby needs to know.
Now on the other hand, is not telling not going to harm anyone? You had a drunken one-night stand outside of your marriage? Have you made changes in your life to make sure it won’t happen again? No STDs? No accidental pregnancies? Are you doing your best to make the marriage work? Then I don’t see any reason to tell your spouse and I agree with faithfool that in a case like that it’s more about alleviating your own guilt than helping your partner any.
When it comes to children though, I generally believe in coming out with the truth, if only so they can find out about their heritage (says the adopted child whose real father died without passing on any information).