There's use and there abuse, true.
But it's also true there's some amount of risk involved with what you perceive as responsible use.
If your personal choices don't interfere with your relationship, and the significant other disagrees with you consistently, then there's a problem there.
But in this case, a personal choice of anabolic usage can be seen to potentially interfere with the relationship in the event that some unfortunate reaction or other mishap occurs.
I'd guess she just cares about you and doesn't want to see anything bad happen. If that's the case, I think it's better to find a compromise rather than drop a relationship just because using steroids will make you more content.