Men's physique and "classic" physique don't require as much muscle as the traditional bodybuilding classes, so surely you can do them naturally, right?
Maybe. At the lowest, local level. And you won't win.
The fact of the matter is that you need a helluva lot more muscle than you think to compete in these sports. In classic physique for example, if you're 5'6" then your weight limit is 170 pounds. At 5'10" your weight limit is 192 pounds and at 6'1" your weight limit is 232 pounds.
Now, this might not seem like that much but understand that Arnold was around 225 pounds at 6'2" in contest shape, and he wasn't as lean as people are getting at now. So a 6'1" guy would actually have to have more muscle than Arnold to exceed his class limit. We all can be bigger than Arnold naturally, right?
Frank Zane was 185 pounds in contest shape at 5'10", almost 10 pounds under the classic physique limit. We can all be 10 pounds larger than Frank Zane without drugs, right?
The issue is that the average gym-goer grossly underestimates how much fat he'd need to lose to be in contest shape. A guy walks in, he's 220, has a small amount of abdominal definition and maybe a biceps vein. In his mind he's 20 pounds away from contest shape.
Well, from my experience, when a normal guy has a full six pack (or full abdominal definition) he still has about 20 pounds to lose to look ready for the stage. So our 220 guy would most likely need to drop down to 175 to be contest ready. An IFBB pro friend of mine competes in the 212 class and at 235 he has full abs and drop down to about 208 in contest shape.
What I'm saying here is that decent physique guys are much bigger than you think. Yes, it's more likely that you could be competitive as a drug-free contestant, but it would still be the exception.