Etiquette? Intent is 9/10ths of the answer.
Accidental boner? Who cares!
Walking around with a semi! Happens all the time and people can just look away. Some are growers some are showers and who knows what a semi is. I would probably like it but not pay it much attention.
Alone and a receptive situation that won't affect others! As long as it's respectful and discrete who cares. I carry a towel so I can cover up anything erect. I don't worry about semis because no one seems to. Generally I leave people alone and they leave me alone. Frequently I get smiles, hellos, sometimes an admiring comment or an approving smile after looking. If I am in a private place and someone smiles or is encouraging like a sauna, I will cover up but occasionally leave room for someone who is obviously interested - but not if others are around.
Raging boner in the shower you can't control? Turn around? It is cool with me. I might even like it but I would just let you be.
Boner walking around on parade cruising? A little rough. Why not carry a towel. If the towel is far away and your not cruising, and you get a boner, still NBD. I would like it and a few other probably would too, but I wouldn't let it get sexual.
Kids around? A consideration but again if it is unintentional and there aren't good options like your towel is far away, I don't think anyone should be ashamed about it as long as they are trying to be avoid unnecessary or sexual situations. (A kid around would kill it for me I think!) I would be more concerned about modeling shame or embarrassment than I would being comfortable with a big or even hard cock.
A place where others are comfortable, like a sports team where it happens and you know people don't care, or a gay gym? As long as people are cool no problem.
It is nice to be nude. It is nice not to be ashamed of the best parts of me. It is nice to be accepted and accepting. Boners and semis just happen.
Creepiness... another matter. Here is a story that bothered me.
I used a university gym as a community member. There were communal showers and solo showers. A lot of guys had sex or beat off in the solo showers. The symbol seemed to be an extra towel over the crack in the door. You could often here pretty discrete sex or guys coming and breathing hard. I was there to work out not whack off. At first this bothered me, maybe because I wasn't as comfortable with my sexuality then or maybe because I was jealous. Or maybe because I felt I needed to wear flip flops. I talked to a few friends about it, and eventually was fine with it.
But at first I used the group shower. Towel rack was far away. A lot of guys with semis with no apparent intent to cruise. Mostly guys just faced the wall and waited for it to go down. That's what I did. I liked showering with other guys just for bonding really, and because I didn't need a solo shower. I liked being nude and it being okay. I found it positive and not sexual.
Then one day I walked into a group shower. There was sn absolutely massive guy there hanging down at least 10 inches. He was facing out and thought everyone should watch him. He was grunting and slapping his dick and looking for attention. He got hard. It was so heavy it wouldn't stand up. But it was bigger than 10. I got kind of pissed off that I couldn't just shower and felt like I had to look away. He was horny and angry that no one else wanted to hook up with him, and he almost forced a reaction. So I just left for the solo showers. I think he just felt like he had a big dick and guys should bow to his authority. But there was nothing cool about him or his behavior.
