The topic of marriage is one that should be approached with the notion that one day she or you may loose interest in, or become unable to do, BDSM. That you are loving what is currently going on is great.

So my suggestion is to broach the topic. "What are your thoughts on marriage?" would be an appropriate opening. Remember communication with your partner is critical in any type of relationship, and being your her submissive, she will want to know whatever is on your mind. It enables her to better Dominate you, and enables you to better serve her.