I think those that object to steroids do it for one of two reasons.
First there are the purists. Basically the people who want to be able to compare Babe Ruth, Hank Aaron and Satchel Paige to any player today. Could Hank Aaron have hit more home runs if he used steroids, or whatever? Probably. I think a lot of baseball fans fall into this category, as they tend to be the ones who are nuts about a player's stats, and whats the point in having stats if you can't compare them.
Then there are the people concerned about atheletes are role models. Essentially these are the ones who don't want college or high school kids taking steroids in the hopes of making it to the big leagues, because they won't. If professional atheletes are allowed to juice, then anyone who wants to play in the bigs will do so too. The problem is that as each level of competition increases, the pool shrinks. Compare the number of people who play little league, to the number who play high school baseball, to college, to the minor leagues to the big leagues, to the hall of fame. With each step you cull like 95% or more, of the players. And since steroids can have massive health effects, it comes down to a public health issue. Now this would seem like less of a ridiculous argument if there were a way to determine which players have the greatest potential beforehand, and then once they turn pro they can do what they want. This might seem like a fantasy, but if you think about it, that's how the clean system works, the best players make it. Allowing players to use steroids, won't make it any easier to get to the next level, since it will only move up the level of performance needed to make the next level.
I mentioned baseball, but this holds true whether you look at baseball, football, rugby, cycling, swimming, running, soccer, etc.