A little leaven leaveneth the whole lump.
This analogy from verse 9 in Galatians 5 is referencing the use of yeast in baking. One bit of yeast in a ball of dough will cause that dough to rise, it doesn’t matter how much yeast or where you put it, the dough will rise. A little leaven leavens the entire lump.
Now, in context, this is not referring to immorality, but it is a fact across the board and the analogy can very easily apply to lust.
Just a little lust affects your entire life. You may think, “Oh, it’s just a little fantasy” or “Oh, it’s only natural; there’s nothing wrong with masturbation” but what is the truth? The truth is our actions affect us, through and through, and a little leaven in one area of our life will impact other areas of our lives.
This makes purity so important. Without purity, we can’t function properly. We can’t worship God purely. We can’t interact with other appropriately. It may seem like no big deal but it affects us immensely.
Think about it, if I had a jug of ice water in front of you but poured in a half-cup of bleach, would you still drink it?