First up, the explanation is evolutionary. According to a study published in the Journal of Reproductive Immunology (Journal of Reproductive Immunology) in 2000, swallowing semen can make your body accustomed to someone’s DNA so your immune system does not act during pregnancy. Since a lot of fertility complications are triggered by identifying the genes of the father as foreign integrating them early into your body will help you out when you are pregnant. This sounds strange but scientists believe that for blow jobs it is a possible evolutionary explanation.
2. "Greater sperm competiotion"
If men go down on women, what? Our parents could have practiced the custom too. One study published in Evolutionary Psychology in 2013 found that men who expected a lot of other guys to be involved in their partner (aka men who had “greater sperm competition”) were more likely to make their female partners orgasm through oral sex. The writers believe that the act can work to prevent adultery, which in effect removes competition from sperm and makes people pass on their genes.
4. Make your partner to feel good
However none of these explications explain why people who are not in heterosexual relationships have oral sex— which clearly happens all the time in same-sex couples. It’s an example of how the reasons a behaviour within a species has evolved are not always the same as the ones why we actually decide to do it. We have oral sex mostly because it feels good or because we want our friend to feel good.