no offense, but you sound like a COMPLETE idiot
you either love him or you don't, for who he is. Jimsbbc said it best in one of his earlier posts. Why are you trying so hard to control him? Why put the relationship in jeopardy because of something you are ignorant of, yet expect him to respond to your ultimatum of quitting "steroids" or leaving him.
So, if he admits it are you still going to leave him? I'd say you were a real cunt if you did that. Why don't you ask for his side of the story (in honesty) if you have any sort of trust between each other you should be able to have an intelligent conversation about his goals with steroids and why it is important to him before you start going bonkers and threatening to leave. in my opinion. that sounds a lot better than your whacked out version.
point being, just talk to him, and LISTEN. For once, be the quiet one, learn to keep your mouth shut, whatever, until he tells you straight out what is going on and how he feels before you tell him what bothers you about him taking steroids. If you actually knew anything about steroids (and hopefully he does too if he is taking them) then this wouldn't be an issue at all.
I don't think you should be worried about having babies with this guy if the thought of him doing something that you disagree with with such enthusiasm (steroids) is simple,, yet enough for you to leave him.
if the relationship means anything, learn to compromise and support your mate.