I am struggling with sexual thoughts and am wondering about the teachings of the Church. Are all sexual sins mortal and 
does thinking of sex without arousal a mortal sin? When does the thought become mortal; is it when we don’t control it or is it always mortal? What exactly is lust and what are the church’s teaching on it? Is it always mortal as well?
Dear Friend, your questions are very direct, which is good, but I am lacking a bit of context. This leaves me with the task of guessing a bit. To try to cover the spectrum of situations, I will first assume the worst, and then in the second post I will answer your questions from a more positive perspective. Also, because these discussions can easily get a bit murky, the need for clarity will override my instinct toward being sensitive to the reader.
Are All Sexual Sins Mortal?
This question is not problematic in and of itself, but it is commonly prompted by a problematic state of heart. When people ask this kind of question they are often asking it in order to determine how to approach a sin they desire to participate in while at the same time attempting to avoid the penalties associated with that sin. Here’s how this question might be restated in this light:
Dear Dan, I want to do XYZ with my girlfriend, however, I don’t want to go to hell. How far can I go before I am in mortal sin?
As you can see, this reveals a person who is clearly aware that the situation they are imagining is in the territory of sin, and that there are grades of sin associated with this activity. This approach falls into the category of grave sin because of the willful nature of the situation, the gravity of the sin, and the fact that it is also a sin that often involves the soul of another. Let me say that again to be sure it sinks in. Taking this approach to this question is inherently grave.
Even if we set aside the grave nature of the sin itself, in this scenario we are not falling accidentally into sin here, but we are entertaining it, judging it, weighing how much we might damage our relationship with God and how we might achieve what we desire with all the perceived benefits but with minimized consequences. This approach is akin to asking “How much arsenic can I add to this soft drink before I am likely to die if I drink it?”
As a setup for the next post, here are a few excerpts from the Catechism on mortal sin:
1857 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.”
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: “Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother.” The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.
In our next post we will make more positive assumptions about your question and explore the idea of grades of sexual sin and lust.
