I am reluctant to join in the current brouhaha about the definition of marriage. Firstly, I regard it all as statist agitprop to trap us into conflicting positions that can only be 'resolved' by the very last thing we need; more state interference. Secondly, I suspect it is a ploy to flush out 'homophobia' so as to give a now entirely redundant 'gay rights' campaign a new lease of life.
There is no good reason for the state to be involved in defining marriage legally. It should not be so much a civil right as a civil rite. It is essentially a personal relationship that can only meaningfully be defined by its participants in the context of their own beliefs and values. The state's current involvement achieves, and its proposed future involvement will achieve, precisely nothing that could not be done better by a combination of civil contract (regulating property relations between the parties) and statute law setting out the responsibilities and rights of parents.
It is particularly amusing that gay people demand redefinition of the current legal institution of marriage under a banner of 'equal rights.' Marriage under English Law is a profoundly unequal institution. If people were as diligent about entering into a marriage contract as they are about buying a house, most men would be advised against and most women would be advised for. Not because the rights of a couple during a marriage are unequal but because of the way the law works on exit.
Be that as it may, as a libertarian I am happy for people to enter into personal relationships of whatever kind they like (and using whatever terminology they like) as long as they take responsibility for their offspring (if applicable) and each other and don't expect others to support their lifestyle choices. If 100 humans want to enter into a mass marriage in whatever combination of sexes and sexual orientations they please, that's fine by me. I only expect them to be able to afford a sufficiently large house and matrimonial bed without recourse to the public purse.
Seriously, I don't care how many are involved. Bigamy would be one of the first crimes my libertarian govenment would repeal. I don't care what sex they are. I don't care what sex they have. My only legal requirement would be that they are of legal age and mental capacity to embark upon their adventure.
Let me hastily pacify shocked social conservatives and people of faith among my readers. I am happy for my religious friends to define marriage their way and for their church to teach that any other way is wicked. Provided, that is, they demand no earthly sanctions for breach of their rules. Given what they believe God has in store for sinners, earthly punishments anyway seem a bit de trop. It is the job of churches and the faithful to evangelise sinners and lead them to the right moral path. The law is (or should be) just there to stop us getting in each others' way. It should certainly not be there to tell us how or what to think.
You may protest (and with good reason) that the law needs to define marriage at present because so many laws discriminate between those married and those not. That problem is simply solved. Neither taxes nor 'benefits' nor legal rights should vary by reference to what is, ultimately, a personal choice. All humans should be equal before the law, regardless of their household arrangements.
So let me answer my own question before turning it over to you gentles to answer it better. Marriage is a personal matter which need not concern me unless it's one in which I am participating. Do what you like. Preach what you like. Accept or don't accept other peoples' definitions of marriage or lifestyle choices. Please take responsibility for your partner's (or partners') well-being and welfare, as well as for that of any children you have with anybody inside or outside your marriage. Please don't expect the rest of us to enforce your view on other - or to refrain from ridiculing yours if it strikes us as amusing.
I am finally old enough to know that the more right I feel I am about something, the more likely I am to be wrong. So please feel free to correct me in the comments.