While human have unusually big dicks, we also have unusually smalls balls. Other manmals are able to breed several dozen times a day, if only for a few seconds at a time. I also don't think big dicks breed more successfully, you're trying to shoot at that uterus, not stab it.
For humans impregnation is the least of our problems, it's bringing up the babys that takes way more time and effort than all other species. I'd argue we're selecting for slowing our reproduction cylce, because historically why were not able to bring even a majority of births to adulthood.
So when you're maintaining a big erection for a long time (the human way), rather than a small erection many short times (monkee) you're probably spending more quality time with your partner, because bitches like it when you look them in the eyes and think of them as people, or whatever. We are also the only animals that sexualize tits, encouraging fucking from the front.
This all leads us to fewer impregnations and more pair-bonding, leading to fewer children but greater investment in co-parenting than other species.