after being in the adult biz for a long time, i got to know know of several sets of adult father/son and brothers, twins, etc who are comfortable being nude, erect whatever.
(dont want to go into lots of detail as it would send this off-topic, but send me a message if u want to discuss privately)
Although from a perspective of an industry that most of us probably have not been a part of the way you have stated it seems mostly on-topic so I think you could share a bit more here if you so care to elaborate a bit on this perhaps unique perspective on this specific topic.