In true Steinem fashion, I think that she might respond to the premise of the question with "Why not?" Steinem has always been outspoken about the need to prevent being placed in a box. Her writings and speeches have demanded the reevaluation of socially dictated positions. When she says, "A woman needs a man like a fish needs a bicycle," it is a statement that a woman can repel the social persuasion to get her married. At the same time, while society pushes, Steinem would argue that women can and have a moral obligation to push back. Additionally, Steinem would criticize the opposite position. Namely, that a feminist woman should remain unmarried. In Steinem's consistent voice of dissent, both socially dictated positions remove the individual's voice. Her decision to marry later on in life reflects a personal choice, something to which Steinem's work has always committed.
Another dimension can be discussed here. Steinem marrying late demonstrates that women who are older can still live life and have a voice in their own being. Steinem would reject the idea that younger women should be the only ones in the position of autonomy. While society advocates youth, especially in the case of women, Steinem's choice to marry later on in life is a repudiation of this. Her marriage is a social and political statement that older women have just as much freedom and autonomy as younger women. The ability to find a soul mate is not limited to the young. For Steinem, limiting older women's voices is just as offensive to limiting any woman's voice. It is in this where one can see a political and social message intrinsic in Steinem's marriage to David Bale.