Dad was from a generation (born 1921) where men who wore boxers dressed right or left in trousers (hung down a trouser leg) and he always showed, but not obtrusively.

Unfortunately, near the end of his life when he was ill with ALS, he needed the condom type urinary catheter, and they come sized. His was the large size.

My two brothers are likely the same, but I only shared a bedroom with a younger brother and saw him hard once when we were teenagers. He was asleep and had morning wood, which was pretty similar to mine. Older brother never revealed.