One concern I have with legalizing behaviors that I view as immoral is that I suspect there is a reciprocal relationship between law and morality. Changing values lead to new laws, and new laws cause a change in values.
Using GSS data, I looked at the strength of the relationship between: 1) illegal drug use and favoring marijuana legalization, and 2) one's view on homosexual sex and favoring gay marriage. Eta-squared for the first relationship is .057 which indicates moderate strength. The gamma statistic for the second relationship is .79 which is very strong. Supporting a law, of course, does not logically require that one is in favor of the behavior in question, but sociologically the two seem to move in tandem.