To be fair, when the succubus tries to seduce Anon she fails miserably at getting him interested in any way. We can safely assume he didn't get an erection at all and, therefore, we are not yet sure that he wouldn't be some kind of epic grower (from Flabby Tiny Prick to Godzilla Dick).
I did a quick sketch to illustrate the idea with random characters so I can better teach my theory which is of the utmost importance:
View attachment 3211573
See? According to this academic and educated theory of mine, Anon might be hiding some huge secret. We never know!