bro... I am sorry to hear about ultimatums.. they are a lose/lose scenerio, there's a good chance it won't be the first time.. and that's not to be rude or mean.. it's the God's honest truth.
Does she work out? Does she diet? Does she surf the internet? I ask, because I met my now wife from here.. she was a mod and helped with female nutrition/training and even 'supplement schedules.' ... the fact is... first of all, you guys DID talk about it.. communication is the best part of the relationship not ultimatums.. and it takes a person with balls to talk to their sig other about this type of topic. Find out what her issues are, is it expense, is it risk, is 'just because?' .. maybe it's because in the public eye there is more negative propoganda than there are facts.. and why? .. because the general public doesn't want children using.. and ya know what.. either does anyone here... lack of education is a terrible thing.. young OR old. Anyone who does/takes anything without knowledge of how it works, why it works, and the proper usage will get hurt/sick... even aspirin!
Point is.. if she doesn't already, it might not be a bad idea to let her into your world.. show her this place, give her the option to partake in your hobbie on your level.. and maybe she might be willing to give it a try.. possibly NOT the supplements, but at least on an educational level she will be able to relate and understand.
The one thing I got lucky with was a woman I never have to hide anything from. This hobby also puts you in a position that when it is part of your lifestyle... it helps make/break the man you are. Did she meet you while she was on? Did/does she like the man that you are? Just things to think about. You two obviously have something good going if you can communicate even on some of the scarier topics in life (could cause conflict).. if they didn't then.. why now? .. and lastly.. since you have talked about it already... I hope she never held your hobbie over you head..
These are just opinions based off of my experience... the hard part is over (first time discussion), but understanding why you have this ULTIMATUM lingering over you head w/o potential fighting is another challenge all in itself.. Try to understand from her shoes as well so she can see that you ARE thinking of her and you want to try to understand her reasoning with an objective point of view...
If the reasons make sense.. then of course you should definitely make a real decision, because if you choose to stay on after saying you won't... she'll know, and it will come back to haunt you.. no ifs/ands/or butts.