I’m going to basically mimic what @syz originally said. My understanding is that a female orgasm makes it easier for the sperm to get to the egg, the muscle contractions carry it along. For a man, really the strongest swimmers are going to be the first ones out, he injects billions of theose little buggers and it only takes one to do the job. It seems like you’re pretty much emptied out for a while…if I recall my teen years accurately, the first time I’d release a load of knucklechildren, the volume would be large, the velocity would be great, and the product would be very coherent. By the fifth occurrence of self abuse in a 24 hour period, I’d be lucky to produce a few drops which barely drizzled out and which seemed to be almost separated, sort of a bright white chunky part in the midst of something the consistency of water. Basically my point being that ejaculate 2 is not as potent, powerful, or aggressively injected as ejaculate one. And that’s after waiting several minutes to as much as a couple hours to “reload”, and after a while, your nutsack is essentially drained, and let’s face it, you probalby wouldn’t want to raise a kid that was produced from cum #5.
For the female, if she can cum before the guy does a few times, it readies the area and makes it very hospitable, then when the man gravy starts to flow, she is ready to carry it on the ovarian expressway. Also, realize that a man does leak a bit before he shoots his load, which is why pulling out isn’t always effective…you can shoot it all over her chest if you want to, but if you don’t take other precautions, you’ve left perhaps a few million little determined critters in there. If she comes a few times, her body can make sure that even with the limited supply, she can get her egg fertilized. In other words, from an evolutionary standpoint, her body needs to do whatever it can to guide those little buggers to the right place, all he really needs to do is fire off as many as he can….why waste bullets when one will do the trick?