Mah Man when he grew Big and Green his penis was stretch out to allow the skin to stretch everywhere else, hell why do you think he turned green? Wouldn't you? Luckily his Butt stayed the same for him. Just imagine hulk without a Butt, LOL

Okay, I am done being an idiot! LOL

Simple - Lycra, also known as spandex or elastane.

Because they didn't want children to see a giant green dong spring out. My guess anyway.