Adultery is a sin. If you ignore this fact and get into adultery you will find that you are hurting your partner and breaking the intimacy and security that marriage provides, you possibly ruin your family, get a divorce and your children will be raised by one parent and then possibly a stepparent who does not love them as their own parent would. These children then grow up without the influence of their father, make mistakes in their lives based on their family situation and the damage goes on and on.
The same is true of other areas if people just follow the results of the behavior that has been condemned.
So, even though they can justify a behavior, they cannot justify away the damage that might be caused by that behavior - some of which is not apparent but may show up much later.