I assume by mentioning "natural law" you are referring to pro-creation.  A man and a woman do not need to be married to pro-create.  Single people (both gay and straight) can pro-create by donors and surrogates.  Gay couples can pro-create by donors and surrogates.  Many married people decide not to pro-create.  So obviously, marriage (same sex or opposite sex) has nothing to do with the "natural law" you are referring to...

So if opposite sex marriage has not gone the way of the hippies, why would same sex marriage?