People have been using their mouths to stimulate each other sexually for about as long as there have been people. How about kissing each other? Men have been kissing women on their necks, ears, breasts, backs, thighs, and anywhere else they can think of that'll arouse them sexually ever since they found out it works. It didn't take long for some enterprising man to find out that kissing a woman's vulva worked, did it ever. The Latin word cunnilingus was coined to describe licking the vulva.
Men were not alone in searching for ways to arouse the opposite sex. Some equally enterprising woman found out that men liked for her to suck their penis, it worked just as well. The Latin word fellatio was coined to describe sucking the penis.
Let's define and use the more common words to describe men and women's genitalia and the various sexual acts. Penis, cock; vulva, pussy; vagina, cunt; Coitus, fuck; cunnilingus, eating pussy; fellatio, sucking cock, blowjob; performing cunnilingus or fellatio, giving head; ejaculate, cum or shoot his load; orgasm, cum or climax; pubic hair, women muff or bush, men bush.
Many men and women, today probably most, use some form of oral massage or kissing of the sex organs as a normal part of foreplay to warm up their partners prior to fucking. They probably don't think of that as "giving head," and I would agree that it's not. It's simply a part of the foreplay used to prepare each other for fucking and hopefully achieving a sexual climax.