"You didn't think I was going to sit there in Whyte's and eat your cinnamon toast as well as my own, did you?" Her voice was pitched low,but it had an edge on it. "The last time you heard a police alarm and walked out of me you left me sitting in a taxi outside City Hall until the Marriage License bureau had closed. I'm not letting you get away from me again that way."
Hildegard did not mean "get away again" from marriage, but from involving her in an investigation. The novel later reveals that they realized that they were not suited for marriage. However, in one of the later movies in the series, they are attending a social event together (with Piper in white tie and tails, no less), so they were still an "item," but just never got married.