First of all, the average vagina doesn’t expand nearly that much. The total depth of an aroused vagina is about 5 inches on average, possibly less. This means that an average penis is perfectly capable of reaching one of the fornices near the cervix. This will be apparent to anyone who pays attention to the specific sensations they feel during PIV.

Second, if you’re having front-entry PIV (missionary, cowgirl, etc.) reaching the posterior fornix with the penis is nearly impossible because the penis is almost always curved upwards, so you will always hit the A-spot in that position. In order for a phallic object to hit the posterior fornix, it would need to be angled in the posterior direction. This means that it is much easier to hit the P-spot with rear-entry PIV (doggystyle, reverse cowgirl, etc.).

Evolutionarily, the vagina has both fornices for a reason: The two most pleasurable spots to hit with a direct “poking” force are the places where a penis is most likely to hit with the tip, so it makes sense that no matter which position you get into, the average penis will typically hit the sweet spot since the penis and vagina have evolved to accommodate each other.

The mentality espoused by this diagram that the typical person with a vagina can only experience the utmost pleasure from an exceptionally large penis, which only a small percentage of the population of penis-owners actually have, is harmful to public sexual health.