Later today MPs will vote on proposals to allow people of the same sex to tie the knot in England and Wales. If the law is passed, will it be a great day for Britain - or a bad day for marraige. If marriage is a good thing, why not open it up to include as many sections of society as possible? Or have we lost our moral compass? Marriage is supposed to be between a man and a woman and opening it up to others could undermine it as an institution.