Dear Curious,
All very good and very interesting questions! Cunnilingus and fellatio have been practiced throughout history, in various societies. In the Kama Sutra, there are writings on how to perform such acts, and also who should perform them. As to fellatio, it was done by eunuchs, some dressed as males and some dressed as females. Women who performed fellatio were generally of lower castes or flat out courtesans. Fellatio in India was "allowed" in some geographical areas and not in others. It is also written that when a man and a woman lie together with their faces toward their respective genitals, it is called "the congress of a cow." [This phrase was also used in the film comedy "Best In Show," however, it was erroneously describing the 'doggie style' position.]
As to why people like it, well, the anatomic explanation would be that you have mesodermic tissue in those areas, full of nerve endings and sensitive to touch. It's the same as when you stick your tongue in someone else's mouth to kiss...yummy, huh?
Now, psychoanalysis has attempted to present a unifying psychosexual theory about everything between men and women. Freud postulated that while there was nothing specifically 'wrong' with these sexual practices, the context in which they were experienced, and particularly during which psychosexual stage, would have a profound influence on the psyche of the individual. Since Freud, many later psychoanalytic theorists have presented their "findings." A conceptualization follows along Freud's first psychosexual stage, namely the "oral" stage. According to Freud, if you were "arrested developmentally" at the oral stage, you would become either one of these two types - an oral incorporative type or an oral expulsive type. So, if you're a gluttonous type (hey, isn't that just about all of us in the US) then you like to gorge on everything..including genitals! If you are an expulsive type, then you're nasty, uptight, rejecting and spitting out all that is given to you...and no, then you wouldn't like to have oral sex.
On a clinical level, I think many psychoanalytic theories have done more harm than good. We are progressing so rapidly in brain research and behavioral genetics, that psychoanalysis is falling out of favor in the mainstream psychiatric community. Could you imagine that only 30 years ago, we were still talking about schizophrenogenic mothers (moms causing their kids to become schizophrenics?!,) homosexuality-causing family dynamics, and various other issues which were deemed to depend on external influences. So at this point, I don't care much for what psychoanalysis has to say about sex in their epigenetic (stage) theories. Personally, I say that I've always loved fellatio because my mom quit breastfeeding me too early, and that the penis is one giant feeding nipple...how about that for a brainshrinker to stew on!
Keep on being Curious to know,
Love Goddess
