It depends on who, when, and where. I'll just go with normal, everyday sightings here, not when someone "accidentally" peaked over the fence and saw it at a backyard sex party, etc., nor what happens at airport security.
Sometimes I'll say, "Oh, it's just a little Daddy Dick."
Other times, I'll have more fun. Once in the restroom somebody said multiple times that it was so huge, but as alcohol might have been involved, I turned to face the third guy pissing off to the side, and asked for a second opinion. He agreed it was huge.
But the most fun time is when this hot surfer dude came in along side me while I was pissing, took a look, and before he even got to the urinal blurted out "That needs to be in my ass!" Three minutes later, it was.
An odd one was sort of indirect, through a third party. Two (gay) guys started cruising me on the patio of a straight bar near my house. I believe they had noticed the bulge in my jeans. A couple of women they knew showed up, and the five of us got a table and had a few rounds. The guys went inside, and one of the women also, to get another drink. I'd had quite a bit of beer, so I excused myself as well to piss. I encountered the two guys in the restroom, but nothing was said. On the way out, I was delayed quite some time at the bar getting another beer. When I finally got back to the table, I found the two guys had left for the night, but the two women were there, and I re-joined them. One of them said to me "He said he saw your dick..." As for reaction, I ended up closing another bar with the women.
At the same bar, different night, the two (straight male) bouncers asked me why there was a little tag that said "Hello! My name is...Huge" on my T-shirt, and then asked about the bulge. To make a long story short, it progressed to me getting out my phone, going to the website of a porn studio, and showing them the first 15 seconds of the trailer for the last video I was in. (I covered the screen right when the the twink started blowing me, I figured they'd seen enough.)
I've had another third-party one, too. I was staying in a motel in San Francisco, and a friend of mine, who always likes to get chummy with the hotel staff, was also staying there. One day he catches me in front, and tells me the desk clerk on duty at the moment wants to see my dick. "He's completely straight," my friend continued. "But he keeps asking me about the bulge he always sees all the times you stay here, and won't believe me when I tell him that it's all just you." So we walk into the office together. "He's going to show you his dick." I look at the clerk, and he nods, so I guess it's OK. So I unbuckle my belt, pop the fly buttons open, and haul it out. The poor guy was wide-eyed and speechless, but I'll take what he requested from my friend as a complement.