Like
@Cappy_Dick said, it's more likely to happen around adolescence (though I'd hesitate to say it's "common"), but past that, it's more a gay/straight or bi-curious fantasy that exists only in the mind and websites such as this one.
Though I'm sure there are super rare instances when adult men might get together for such things, but those men are either gay, bi-curious (if it's the first time) or bisexual yet choose to still call themselves straight (if it's a repeat activity) and more likely to be a one-on-one, then group thing. I think there'd be more of a chance of getting stuck by lightning on the second Tuesday of the month then a group of "straight" friends getting together for weekly wank sessions.
I've never as an adult hung out with friends and we've all gotten so horny (for whatever reason) that someone suggested we pull out our dicks and stroke in front of each other. Porn is made to show fantasies that people think about, but in real life never actually happens.
