I have a new hygiene theory.
When you’re a guy and you go to the toilet, you need to touch your genitals to point yourself in the right direction. When you’re done it is common practice that you wash your hands after (although studies show that about 20% of adults don’t). My theory is though, for the average person, the hand is actually less clean than their genitals.
Have a think about it, all day you walk around touching grubby door handles and hand rails then you go touch your genitals when you go to the toilet, clean your hand then go handle some germ covered money, open bin lids and go touch yourself in the toilet and make your penis even more germ contaminated. Then you go home and you want your partner to put it in their mouth.