A male dog hikes a back leg to mark a tree. Instrinctive impule to mark territory.
Men simply mark wherever the lady will let him shoot; face, tits, mouth, ass, pussy. It doesn't matter where so long as its on her. It's a power trip but don't think for a second that it's not also a primordial territorial marking of property.
The ironic thing about it is that once the man empties his nuts the woman is supreme since she can go for another round so quickly while the man needs time to recharge.