I think most people wouldn't walk in on someone who's stroking, on purpose, 'cause they figure he's having a "private moment." As for whether any given guy would find it hot - that depends on the person.
The only way to get to the laundry room or back porch in my building without leaving the apartment is to walk through one of the bedrooms. When I once had a tenant who rented that bedroom, we worked different shifts so privacy was never a problem. This guy had a big wicker wastebasket between his bed and desk, clearly visible to anybody who walked in. Every time I needed to get to the porch or basement, the pile of crusty tissues in that basket was higher. He never had a cold, so guess what the tissues had cleaned up?
The thought of him pullin' it, let alone being invited to join in, never excited me.
Upstairs from me live two straight men, brothers, who are in their 20's. One is HOT, the other very cute. Not infrequently does one or the other of them host a female companion and go at it with her 1-3 times in a night. Our 1908 building has noisy floors and no insulation, meaning not only do I hear the bed creaking but I also hear the chick's "sexual responses." Now and then the guy's loud enough to be audible as well. What's the effect on me? Snickers and rolled eyes. HOWEVER! Every once in a while the guys wrestle each other, either for fun or to settle an argument. Furniture crashes around if they're inside. If they're on the back porch you can hear them colliding with the railings or rolling on the floor. When one gets the other into a submission hold - causing the winner to grunt and bark triumphantly while the loser gasps and cries out - you can be damn
sure my peter's pumpin'.