Preface: The use of steroids as a booster for athletes is dangerous. It is harmful to their health and has unwanted side effects far out of proportion from their "benefits" to mankind. Steroids have legitimate medical uses, but this isn't one. But that's not what this post is about.
Allowing athletes to artificially enhance themselves to become better performers seems to many like a cut-and-dried question, but it's not. Enhancements can be arranged in a spectrum, and where the "dividing line" is along that spectrum varies over time and by situation.
The kind of intensive training, starting from a very young age, that is common to athletes in some sports today, would have been seen only a few generations ago as being absurd, a way to "ruin the game" by artificially inflating performance levels. Most sports now have equipment being used which far exceeds what was available decades ago. Trainers now use computer analysis of motions of athletes to achieve tiny, but in some cases crucial, improvements in performance. Athletes manipulate their biochemistry and metabolism to a very precise level, using frequent blood tests and other indicators, through the use of techniques ranging from diet adjustment to time spent in hyperbaric chambers.
We are long and far away from the point where the plot of most sports movies (where a regular guy with a little talent and a lot of gumption can make the difference in the big game) is more than an exceedingly unlikely fantasy. The use of steriods doesn't really change that. It's just another step.
That doesn't mean there shouldn't be a line. As things change the game has to adapt to the changes, and during the period before each adaptation, those changes will need to be "over the line". When someone has a prosthetic eye that can track the movement of a 90mph baseball precisely enough to hit it every time, or a cybernetic arm that hits them out of the park on every hit, those will be "over the line" until the game can adapt to that. If tomorrow someone developed a steroid-like performance enhancement drug that didn't have unwanted health and other side effects, sports would have to adapt to that, too, and until they did, it might well be best for it to remain forbidden.
But eventually, these things will simply become a new baseline, just like high-performance teflon suits changed the baseline in swimming, and someday, genetically engineered higher-efficiency alveoli will change it again. Human capabilities change; human pursuits must follow.