really hot male lab assitant measures it closely

Elastic ring that can measure how much it streched.

Probably some sort of pressure cuff that they measured changes in air pressure, or something similar?

It’s basically the same thing as a ring fit adventure, a strain gauge, though research ones are far more sensitive

These things are used in really sketchy therapy like conversion therapy. They were also used in pedophilia treatment in the 90s which was basically conversion therapy to track outcomes, basically exactly as what’s written here (though they wouldn’t present actual illegal material, just risqué material like pictures of kids at the beach or something). Penile plethysmography is the area and it’s still practiced in some places, though it’s pretty controversial because they’re evaluating sex offenders for risk of recidivism and viability of release which is always inherently controversial and other more obvious reasons

That's what I was wondering for quite some time for developing a recreationa..... aaaaah medical device using an esp32 that can control other hardware over Bluetooth. A sttetch sensor or pressure cuff might work, but it might be nicer not to have it work like a blood pressure measurement machine.