The only explanation I can think of is that marriage, in the eyes of the proponents of these referenda, is not only between people of differing genders, but also hierarchically places one of those genders as dominant over the other. Otherwise, you'd expect the wording to be about half one way, and half the other.
Not saying these are my assumptions, but if one starts from these three premises: 1) marriage is between dissimilar genders, 2) there are but two genders, and 3) these genders are equal; then the probability that each and every one of these 31 referenda happened to list "man" first would be 1/2 to the 31st power, or 0.00000000046, or put another way, 1 chance in 2,147,483,648.
Sounds a lot like a coin with "heads" on both sides.
I think it's safe to conclude that the drafters of these referenda don't abide the second or third premises.
If you pay much attention to the persecution fantasies of the religious right, you'll soon see that a great deal of their anxiety about similar gender marriage isn't about gay or lesbian couples at all - it's never long before they claim that mothers and fathers have distinct and separate roles to play in raising children. And questioning that really threatens their view of the world.
Of course, another way to look at it is that is you want your heterosexual marriage to be on a stronger and more egalitarian basis, you'd be voting against your own interests to exclude similar gender couples from the institution.