My friends talk about cocks and sex and there is nudity almost every time we all get together. Big dicked guys are more likely to show off their junk as well.
To clarify you are specifically talking about gay/bi men, correct? I don't think that's common with straight men. I've only briefly discussed penis size with two of my friends ever (and we weren't celebrating size), and I've never seen another penis in real life except for a few flashes in the locker room once in a while. No one has ever bragged to me or tried to show me anything.
To be fair I have to admit I now recall one friend of a friend, who did claim that a large penis was one of her primary sources of attraction to her fiance. In that case I suppose his size was being celebrated. But guess what, he has to go weeks without sex, months without head, and she doesn't want to contact his semen at all. She also disrespects him constantly, even in public. And she is a former slut with an STD. I wouldn't say his large penis got him to a good place. I also recall one other friend who said her boyfriend had a big dick and it hurt, and there was no mention of pleasure of any kind. She was actually complaining because it was a hassle to have to use special lube every time, etc. That was a case of a woman who was actually unhappy to have a large penis.