Ethics mean we don't know what the average human male erect penis size is.
No, really. The ethics of the studies say that a researcher can't be in the presence of a sexually aroused erect penis. Having the testee measure their own penis is prone to error. There are ways to induce an erection with an injection, so they use that.
Is the size of an induced erection the same as a sexually aroused erection? Probably in the same ballpark, but we don't really know.
Source: Dr Nicole Prause, neurologist specializing in sexuality, on Holly Randall's podcast.
It is acknowledged that some of the volunteers across different studies may have taken part in a study because they were more confident with their penis size than the general male population.
The study I linked seems to include both self stimulated erections and erections due to injection. They also limit themselves to clinical measurements. They mention self measured results but point out that they are unreliable, as you said. They do point out however that there might be a difference between self stimulation and an erection with a partner.
But all in all, there isn't a barrier because of the ethics involved in touching a penis and masturbation.
Who is telling the truth. My ex said it was too big. The bell curves I've found have said "uh what lmfao no way are you that big" but every self reported study says I'm small
How the fuck am I going to ever find a toilet that is comfortable to use in my own home
It's a problem for men with penises that are long when flaccid. Their penises can touch the inside of the bowl when they're seated unless they hold their penis up.