You have to be married first (in a non-same sex marriage). Then, after you're married, use the "Come Hither" expression on your spouse, go to bed, and choose to have unprotected sex with your spouse. The "Come Hither" expression can be bought from the Bowerstone Bookstore .
If you're not married and choose to have just have sex with random people, you will not have a child.
[Maybe it's Peter Molyneux way of keeping your hero from becoming poor after paying child support to all of your...ahem...conquests]