There is a good number of factors going into it to be easy to determine: sperm motility, count, quality and quality of the seminal fluid, cervical mucus acidity and fluidity and chemical compatibility between the two are a few.

Then there is the sperm competition, as sperm cells of different donors compete between each other.

And then there is the mechanical action of penetration, that, considering how apt the penis is to perform the action, pulls out as much as there is inside possible.

Oh, and the more prolonged/frequent intercourse is, it may contribute to lower ph vaginal levels, which is not good for sperm life expectancy.

It boils down to evolutionary competition.

But one last thought: run a train? There are less judgmental ways to say the exact same thing.