Do you know when a mammal is conceived, the embryo is sexless???
Of course you know that.
Do you also know that this sexless-embryo has tissues that can be used to either develop a penis or milk glands (breast)???
Perhaps you know, perhaps you don't.
What decides the development of either of these things is the release of the sex hormones.
The moment it is released, the male developes a penis and is left with useless nipples.
On the other hand, the female developes milk glands (breast) and is left with undeveloped penis.
The next time you're giving someone a 'head', know that you're playing with an undeveloped dick!!☺ 😂