Quite a scope of responses.
I was in aikido before I met my wife, so when we began our relationship, it was quite simple. I made it clear to her that if she ever asked me to choose between her and aikido, it wouldn't work out for us. In response, she required the same of me and her duties as a volunteer firefighter/EMT.
We clearly understood one another's priorities and that what we did was a part of who we were and what attracted us to the other person. Later, without being asked, we have each made willing sacrifices for one another when appropriate.
I have a feeling most people don't address this sort of issue in their relationships right up front and immediately.