You're framing this as a dichotomy between slippery slopes. We must either pretend that all sins are equivalent and thus give cover to the worst sins or we must use the fact that some sins are worse than others to allow the "better" side to backslide. The obvious alternative is to hold all parties account according to the severity of their guilt, and demand that everyone does better. In other words, the guilt of one party doesn't absolve the other or (as children understand) "two wrongs don't make a right".