To vote against same sex marriage, isn’t to vote against gay rights

When politicians canvass for this amendment on the doorsteps they will give us a lot of “what’s it all about”, ie it’s all about tolerance, inclusiveness, equality. They will try to avoid saying exactly what it is — the changing of the meaning of marriage in the Constitution in order to provide for a two-man/two-woman marriage.
Up to 20 years ago, or less, this very notion would have been considered absurd or even risible. How has it come about that what was absurd then is merely not absurd now, but is regarded by many as positive and healthy?