It used to be, in more promiscuous days, that you slept with someone then became friends. Now it's more fashionable to be friends first, then sleep together. In many ways, dating friends is practical: you have things in common, opening lines are not a problem, and you're rarely at a loss for words. In addition, if it's any kind of real friend, they know about your mother, psychiatrist, husband or wife already. They've no doubt heard you let loose more than a few gaseous blasts. In fact, dating a friend is so convenient that it's often possible to skip the date altogether and simply have sex.
A word of warning, however. Friends are harder to get rid of than are regular dates. If you date a friend, then need to dump that friend, you can move away, or you can sleep with the friend's mother, psychiatrist, husband or wife. Any of these, or combination of these, can be effective.