I've been naked around my older brother since young. We had erections around eachother then and still 50 yrs later we still do. We sometimes joke about it but still nice to see. Myself still get erect knowing we will be nude together.
I have the opposite problem. Even though I've been a naturist for a long time, in social situations I tend to shrink up to nothing. I have seen a lot of other men do the same, always trying to play with it to keep it from getting small. I get hard when I know somebody is looking at me, or sometimes if I'm really comfortable with the people (or person) I'm with. I definitely get hard if I'm turned on and hoping to get action.
In the super-straight Army barracks shower room at Fort Jackson in 1962, if one guy popped a boner, almost every guy in the room got one. Lots of laughing and bonding.
Boners are contagious!