Can one have children while practicing sexual magic?

Yes, it is possible to have children while practicing sexual transmutation – however it is not something to be expected. A single sperm can easily escape without the man needing to ejaculate millions.

It occurs where children are within the karma of the couple practicing, thus a child is the result, yet it does not mean that the child will be "special" or more spiritually inclined than any other child. There is a practice outlined in The Yellow Book by Samael Aun Weor which can result in the birth of a self-realized Master, yet this takes a supreme level of discipline and the parents must have the correct karma and dharma. You can read the relevant chapter here: Children of Wisdom.

The desire to have a child is not an acceptable excuse to stop practicing sexual magic. We have heard the story where the wife of a couple who had been practicing sexual magic for many years desperately wanted a child. The husband eventually gave in and fornicated just once with his wife. Because of this a child was born, yet, the husband lost about 10 years of work upon himself. His state of mind, his ability to meditate, etc., were taken away; he was as if he never worked upon himself. Therefore, let those who wish to enter this path seriously contemplate what they want in life. Everyone is free to do as they wish, we only wish to state that the path of the self-realization of the Being cannot be traversed if one spills the sexual energy.

Also note that a pregnant woman must wait for the birth of the child before recommencing her work of sexual transmutation.