Why don’t Bruce Banner’s pants rip open when he turns into the Hulk?
The reason is that the Hulk Comics/Cartoons/Movies are all made in focusing children, so they don't want to show Green porn.
Anyways below are Lee's funny answer on the same topic -
Q: If it weren’t for the Comics Code, would the Hulk’s pants have ripped off like his shirt?
Stan Lee: I guess it probably would have. So occasionally the Code did some good things.
Q: Did you ever try to make sense of the Hulk’s magical purple pants? Why did they always conveniently remain intact while the rest of his clothes were ripped to shreds?
Stan Lee: I just figured that Bruce Banner had probably been a friend of Reed Richards [Mr. Fantastic from the Fantastic Four], and Reed had given him some elastic trousers. There’s an explanation for everything, but you may not be technically advanced enough to follow me on all of this.
They show us Bruce Banner buying some really "elastic" pants in The Incredible Hulk. That's why they don't tear off I believe.