So, what, doctrines should never change? There's some pretty apt examples of the church saying "whoops" and changing its mind, depending on your denomination.
But for the most part we aren't talking about official changes in Church doctrine. (Yes, I know a couple have, but we're talking more about individuals.) We're talking about people justifying this stuff with cute little platitudes like "God is love so whatever I do is fine." or re-interpreting Scripture passages to make it sound like certain sinful behaviors aren't sinful at all and never were.
"He who takes offense when no offense is intended is a fool, and he who takes offense when offense is intended is a greater fool."
"Don't take refuge in the false security of consensus."