There is a difference between not illegal actions and actions that are codified as legal, such as abortion or Gay marriage. Actions that are codified as legal implies public approval and as such enables the participants in the codified action to claim duress by those who do not approve of the action.
To support legalization of Gay Marriage to offer approval. On the other hand, you could support Gay unions with all the same rights and benefits as marriage, without really approving of the actions of homosexuality.
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