Trust me, I'm a big fan. But even so it seems like a somewhat illogical thing to do instinctively. You could even say that society partaking in this act currently is rather weird.
So is this something that humanity has only started doing in the last 'however many' years or have we always been horny bastards and this is something that we have always done?
Sorry, just popped over from /r/askscience. This seems more like an anthropology question, or the premises at least touch on biological anthropology.
FWIW lots of animals orally stimulate one another's genitals before mating. So the premise that humans invented "oral sex" or that they did so recently seems dubious.
Bonobos (genus: pan) and humans (genus: homo) both engage in this behaviour and that might be indicative that the joint pan-hominid ancestor (approx 4-6million years ago) may also have displayed such a behaviour. The evolutionary assumption being that traits shared by 2 related organisms are likely present in their joint ancestor. Though of course we should mindful that not all behaviours may be [directly] genetically coded. We certainly see that many primate lineages do these activities, so perhaps the behaviour is rooted in the joint ancestor of all primates (~25 million years ago).
Broadening our view we see pre-copulatory oral stimulation behaviours in rodents, bears, porpoises, canids (dogs & wolves), ovines (goats & sheep) and even some insects. It is possibly a very, very anciently rooted behaviour but it is largely constrained to a variety of mammal lineages. And mammals emerge about 160million years ago.
Here are two such publications:
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0007595
https://www.nature.com/articles/srep25128
I don't know that this is especially illogical. For mammals it is likely pleasurable and more broadly may aid successful copulation by inducing the necessary receptive state in one or both partners. Of course there is a question about whether pre-copulatory, "preparatory" behaviours are the same as we would consider "oral sex". And we should be careful projecting human social behaviours/constructs on to animals. Few animals engage in sex acts that aren't for procreation. That said sex, and oral-genital stimulation, for only "pleasure" or social bonding is seen in some animals and I can think of at least bonobos, humans, porpoises and other cetaceans (whales).