In the Catechism of the Catholic Church (2352) it says that those determining moral culpability must take into account if a particular sexual impurity is committed because of “force of acquired habit.” How can you prevent people from using this as an excuse to continue in these ways and/or justifying to themselves that they are not committing a mortal sin?
Once again, we must remember that mortal sin has three components: grave action, full knowledge, and full and free consent.
For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent” (CCC 1857).
If any of those components are missing, there is not a mortal sin. A person who commits an objectively grave action with consent that is not fully free, perhaps because his consent is impaired by a compromised willpower, commits venial sin:
One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent (CCC 1862).
The sexual sin the Catechism is considering in paragraph 2352 is always and everywhere a grave action; however, an individual’s personal culpability for that grave action may be diminished if, as one among other possible mitigating factors, he has become addicted to the behavior. Acquired habit, in this case, is a polite term for addiction. For more information on culpability for addictive sins, please see the link below.