Aren't you smuggling in the conclusion? Incremental change that builds on previous changes is generally called a "slippery slope" only when the consequences are undesired. Is the gradual increase in homosexual rights good? Then it won't be called a slippery slope.

More generally, there aren't many changes that "most everyone can agree was clearly not-bad." If everyone thought it would be a good idea, society wouldn't have been doing things some other way.