As seen in the New York Times:
The civil union law was written under pressure, in response to a directive by the State Supreme Court seven weeks ago to assure that gay and lesbian couples are guaranteed the same rights and benefits as married heterosexual couples.
The court left it to the Legislature to decide whether gay couples should be allowed to marry or placed on a separate, parallel track. Both houses settled on the civil union route, sending it through in just 10 days from introduction to voting.
Republicans in the Senate tried but failed to amend the legislation to define marriage as the union of one man and one woman, so a pathway still remains open for allowing full marriage for same-sex couples. Many legislators said they hoped and expected to enact a marriage bill eventually.
I do have to commend the New Jersey State Legislature for their practicality — they sought to get the option on which there was the most consensus passed as quickly as possible, while leaving the more contentious questions to be debated later.
I can easily imagine that there was a concern among the pro-marriage/union crowd that the whole package gould have been allowed to drag on while the question of a difference between “union” and “marriage” was hashed out.