It's my understanding ( could be wrong ) that gay couples have exactly the same rights as hetero couples anyway so I don't see the validity of that argument .
Surrogacy , ivf ( at tax payers expense ) is widely used by hetero couples so I get that argument either .
As far as I can tell " marriage " is simply a term now i.e. it doesn't make any difference to the relationship legally or any other way . I'm puzzled why the gay community has fought so hard for it ? I'm surprised that they didn't want a new term to distinguish them , a bit like being a partner ?
