I thought it was because apes fight by grabbing and tearing, so with that evolutionary pressure applied you basically end up with the minimum effective protrusion.
Also, this is why I plan to avoid fighting apes or at least try to wear some good pants if I do.
You may be right. If there was a selective pressure against large penises due to intra-male fighting, and the lack of a strong enough pressure for larger penises (due to the relative absence of pair-bonding), that would support the observation that gorilla penises are small. For humans, where that sort of fighting doesn't happen, and where pair-bonding is important, penis size would naturally drift upwards until they become too big, or they take too much energy to grow/utilize.
One way to test this would be analyze the fighting techniques of various primate species, and then bin them based on their penis size and relative monogamy. If all primates that grab and tear have small penis, irrespective of their pair-bonding, then perhaps the former is more important.
Also, this is why I plan to avoid fighting apes or at least try to wear some good pants if I do.