so prove it.
Gay marriage has been legal here in MA for five years now and no additional "leaps" have been made. Perhaps you can back up your assertion with some facts? Nowhere that gay marriage is legal has such a leap occurred. Gay marriage is legal in Canada, Sweden, Norway, the Netherlands, Belgium, Spain, South Africa. Four U.S. states currently allow same sex couples to marry, and two acknowledge same sex marriages from other states. Have any of these places "taken the leap" to allow ANY two people to marry?
Your argument can apply to any couple. "If we legalize interracial marriage, there's nothing to stop the leap to interspecies marriage."
You'll have to give us more than a knee-jerk slippery slope argument. You need to prove such statements.

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HM Customs & Revenue have extended married dependent status to second and third and fourth wives in the United Kingdom. Similar rules are being drawn up to cover the rest of the EU. HMCR took this move to accommodate those coming into the UK from countries that practiced traditional polygamy. This recognition, in law, points the way to what the future will look like. Enjoy.