[PartiallyTyped]

> but how do we know one evolutionary theory is the correct one

IMHO, the one that works is the one most likely to result in increased gene propagation even if the percentage is minuscule in the absence of orthogonal genes.

The argument is not that X evolved to do Y, but rather X resulted in increased gene propagation as it happened to do Y.

In the example above, the head of the penis is shaped as such because that shape enables semen displacement. In the same vein, males tend to lose an erection, and therefore the [flaccid] penis loses the displacement property, after excretion because that results in not displacing one's own semen.