the key word is "willful."

no one is saying that Christians are perfect. but when you have a book that clearly tells you what to do and what NOT to do, you are obligated to TRY to follow suit. however, many Christians don't seem to try or have any conflict about it; they just rationalize it the same way you did: "yeah, i know this is a sin, but i know God is going to forgive me, so i'm good."