Is it harmful to lick a woman?

This depends. Under normal conditions, between two healthy individuals, it should not be harmful to lick the clitoris, or perform oral sex on a female (known as cunnilingus). Both individuals should be tested for Sexually Transmitted Diseases (STD's) in order to make sure that neither could pass a disease to the other. In the case that a test can not be performed, it is advised that a dental dam be used. This is similar to a condom, but is instead used while giving oral pleasure to a female. This will not guarantee in any way that either individual will be safe from sexually transmitted disease, though it lowers the risk. Both individuals should abstain from oral-genital contact, if either individual has open sores on either their genitals, or in their mouth.

Again, assuming both individuals are healthy and disease free, there should be no problem with cunnilingus (oral sex performed on a female), including contact between the tongue and the clitoris.