Why two? Civil marriage should be a normal contract between n entities that are allowed to engage in contracts. It seems extremely inelegant to have arbitrary restrictions on it.
I think it's pretty straightforward to come up with reasons why it's important to limit marriage to as few people as possible. One strong one being that it could be very bad for society to allow wealthy men (and women, though I think it would be less common) to marry a large number of women, especially due to the negative effect such a situation would have on lower class men.
But it's not limiting that. People are already free to do so and some religious or philosophies encourage or make it highly desirable. They just cannot be legally recognized which is pointlessly discriminatory.
