What does marriage mean? How is marriage defined? Once the concept of the man, woman and [possibly - not every married couple has children] child is not used to define marriage, then can it be anything anyone wants?
What about a bisexual woman who would like to be married to both a man and a woman. Bisexuals have rights too, don't they? What about polyamory? As long as it's consensual then it's okay, right? How about a marriage among three or six men?
How about a situation where a 42 year old man would like to "marry" a 13 year old boy? How about polygamy, that form of "marriage" has been around since the Old Testament, so why not bring that back?
Every child deserves to have one mother and one father. Just think of the ensuing chaos that could happen if societies wipe out the definition of the one man one woman marriage paradigm.
Why are homosexuals adamant about re-defining marriage? Why can't they call it a partnership or cohabitation? If the aim is to get benefits, let them have the health benefits and the probate benefits.
Marriage between one man and one woman is the basic foundation of society, to re-define that is to invite chaos. Probate laws could be altered to encompass cases involving partnership and cohabitation. Change the probate laws and change the benefit/employment laws, but don't call it "marriage."