"When large populations of male rats are tested on sexual activity during four successive tests, over time individual rats display a very stable sexual behavior that is either slow, normal or fast as characterized by the number of ejaculations performed. These sexual endophenotypes are postulated as rat counterparts of premature (fast rats) or retarded ejaculation (slow rats)."
Since this is clearly not psychological, and conserved in other species, it makes me wonder if in fact evolution hasn't positively selected it. I could see it being a survival advantage in an illicit encounter, since it leaves more time for flight. All this to introduce the best quote of the week:
She said men with primary premature ejaculation tended to be fast reactors generally.
"These men have very quick reflexes. They may be excellent at playing tennis or computer games, for example."