I think its important to go back to first principles and note that marriage is first and foremost a 'contract'. And one of the principles of contract is that because contracts are voluntary agreements, if the two parties wish to terminate the contract, they have the option to do so or to create a new contract. However there are restrictions on this if third parties are involved, and I think it is for this reason that traditionally marriage is considered to be a very restrictive (difficult to terminate) contract because it is assumed that there will be third parties (e.g children, grandchildren, in-laws, etc ) affected by the termination of the contract
It is hard for me to see then, why the coming together of any two ( or even more) people for the purpose of mutual friendship etc. in a sterile relationship that would not normally produce third parties (children) would require a contract as legally binding as that of marriage unless there was an intention that third party issues (e.g children) would be involved.
Traditionally it is a contract, hence why parents would get together and discuss who their children would marry, irrespective of who the child wanted to marry or was even at an age where it thought about marriage, and a dowry would change hands. However, traditional marriage ended a long time ago as did the notion of a contract, at least as a social concept. It would be difficult to argue that for anything but a very small minority marriage is a commitment between two people to spend the rest of their lives together.
The issue of sterility and having children is too easily dismissed as sterility is not restricted to homosexual relationships. If naturally having children is a condition of marriage then I have several hetrosexual friends who would need to have their marriage annulled. The problem is that as a civilisation we have changed marriage several times to the point we are at now where it is a legally binding commitment in love and monogomy, at least I hope people do not propose by saying,
"Dear significant other, I would like to suggest a voluntary 'contract' where, if we, the two parties, wish to terminate the contract, we have the option to do so or to create a new contract."
As you categorically state it is a voluntary agreement. However, I reject the notion of going back to "first principles" as we can't, we can't go back to the prehistoric notion of marriage as civilisation developed. We've tried and we still have no real answer for human monogomy. We've no accurate record until the 12th century of what marriage meant. And even if we take that definition, well then we can also have arranged marriage, child marriage and until very recently in our culture legitimate domestic abuse within a marriage (as in marriage defined where the female is subservant). The first principles argument also has no place, because we cherry pick which bit of the first principles we wish to use in an argument.
The concept and definition of marriage has changed. Thankfully it is now a voluntary agreement between two people who wish (at least at that point in time) to commit to a relationship together for the rest of their lives. There is nothing at all that should or could exclude same a sex marriage couple in this committment. Nothing. Notions of what you believe to be traditional or "first principles" and notions of sterility are not reasonable arguments against why a same sex couple cannot have the same recognition by the state of their commitment.
This referundum isn't seeking to disolve the entire notion of marriage, just permit the use of that term for all who wish, just to allow anyone who wishes to make that committment have the same standing in the eyes of the state and the same rights. It has no effect or impact on all those who are currently married. It doesn't weaken their marriage or commitment, it will just make more people in the state happy and equal.