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.
Sure, they exist, but they have the flaws outlined above.
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.