Why do we keep the sacred separate from the profane? So we don’t dirty up the clean things. The clean things won’t make the dirty things any cleaner, but the dirty things sure can mess up the clean things. So we keep them separate. If we don’t, we’re left with nothing but dirty things. And this holds just as true for concepts as it does for objects.
You have a choice whether to participate in the public sin of others. If you join it, you show others that you approve of that sin, and you encourage them to do the same. Which is why Christians must refrain to the best of their ability from participating in gay weddings.