Copyright infringement, as I understand it, has to do with taking something and claiming it is yours (e.g., you take my sheet music, put your name on it, and sell it as yours).
Then you don't understand it. Copyright infringement is simply unauthorized use of copyrighted material, e.g. making copies. What you're talking about is plagiarism.
Wait, kill the concept?
Anyone want to buy a million monkey-sized coffins?
You bring up murderers as an example, but a person convicted of murder can still get married, right? Where's the proposition outlawing marriage for convicted criminals?
The question isn't whether the Bible considers homosexuality a sin, but why Christians have decided to focus almost exclusively on this one particular sin.
You could simply pass a law stating that gay people in a civil union can call themselves "married" on any sort of official document, such as income tax forms.
So instead of letting gays get married, we'll let them get married!
Beware of the Turing Tar-pit in which everything is possible but nothing of interest is easy.