Was it common for people who grew up in the early 50's, relatively "square" people such as my parents, to engage in oral sex? Sorry, but I just can't picture it. I also seem to remember that it was considered something of a taboo.
Historically and anthropologically, oral sex is nothing new. However, social acceptance of fellatio (oral sex performed on a penis), cunnilingus (oral stimulation of a vagina), and analingus (use of the mouth and tongue to stimulate the anus) has evolved. In the United States during the 1940s and 1950s, despite the outward "squareness" exemplified in television shows like "Leave it to Beaver," the sexual revolution of the 1960s and 1970s was brewing. For instance, it was also during this time that the first issue of Playboy was published and Dr. Kinsey was conducting his now famous studies on American sexuality.
But stepping back a bit further, religious and cultural institutions have often contributed to the negative reputation that oral sex (and sex in general) has received. In the fifth century, Augustine declared sex to be immoral unless it was for the sole purpose of procreation within a marriage. Abstinence and celibacy signified higher spiritual devotion and any sexual act outside of the moral code was punishable. This notion of sex as sin continued in Western society with little challenge until Sigmund Freud brought issues of sexuality into the public realm in the twentieth century, asserting that sexual acts such as masturbation were actually healthy behaviors. This paved the way for Alfred Kinsey, who blew (excuse the pun) the door open with his findings that oral sex, homosexual sex, and masturbation, among other acts, were much more common across the US population than many people would have liked to have admitted at the time.
Whereas oral sex had once been categorized as a perversion, throughout the twentieth century it eventually became seen as an accepted component of foreplay and then a sufficiently pleasurable act on its own. In short, oral sex was probably considered a taboo in the US when your parents were young, and although it has become more normalized in the twenty-first century, people could argue that a taboo still exists. It may feel uncomfortable to imagine your parents in flagrante, but oral sex is a natural behavior that humans and animals have been doing for thousands of years, regardless of its social acceptability.
Though it's entirely possible that your parents have never experimented with oral sex, you may thank their generation for coining the phrase "giving head" which came (again, pardon the pun) into the English vocabulary in the 1950s. Take that, Mrs. Cleaver!Alice!