The researchers used a silicone vagina and a latex penis to mimic intercourse. They were able to prove that "the shape of the penis allowed it to scoop out a semen-like substance," says Dr. Ackerman. She adds that 90 percent of the cornstarch and water mixture (the makeshift semen) was removed with just one thrust.

The displacement theory is reasonable. It could be displacement of bacteria, and everything else.

It's evidence, but this can't be "proven".

Is there evidence of a consistently similar shape among other mammals?

Seems like a reasonable evolutionary advantage, over any shape that plunges additional material into the cavity.