No good Catholic claims they are perfect and free from sin, so there goes your straw man right there. But Jesus said that divorcing and remarrying was adultery, and remaining in that sin is not repentance. And telling people that continuing in the sinful state of adultery is perfectly fine is not merciful. This is not difficult to understand.
“But Jesus said that divorcing and remarrying was adultery”
If you only take one piece of scripture and not the others, that is the result.
Do you ever look at another with lust?