Is the thought of committing a sexual sin itself a sin

Last night I was tired and on my tablet when either the thought or worrying of looking at something sexual or the temptation. I told myself that I wouldn’t and didn’t end up doing so, but then realized that I thought it over. Was thinking it over or about it at all a sin in itself I should worry about, even if I didn’t do anything?

A Stray thought about sex that you dismiss or reject is temptation not sin. You thought about it then rejected by your will that you did not want to do this sin. To be mortal sin it must be grave matter and you are willing it to happen. If you entertain the thoughts and let them go on with it even when you recognize what is happening you are committing sin. Always ask God for help in avoiding sin.

ewtn.com/expert/answers/mortal_versus_venial.htm

See EWTN site for an explanation between mortal and venial sin. When you do commit a sin, we all do, say an act of contrition, tell God how sorry you are and then go to confession as soon as you can.
I’ll pray for you.

This sounds like scrupulosity. You should visit with your confessor for guidance.

This does not appear to be a sin. Let’s compare your example to another possible scenario. Let’s say an obese person who was trying to lose weight was at a restaurant - saw a hot fudge sundae on the menu and thought ‘oh man that looks incredible, I want to order that’ - but then decides to order a salad instead. That person would most certainly not be guilty of the crime of gluttony.

Somewhat different.

Why?

Because mortal sin in terms of looking lustfully, having lustful thoughts or desires - is already grave matter for mortal sin.

The question is - did a person do so with full knowledge and complete consent. Temptation is one thing - mortal sin is another.

Not really.

“Thoughts” and “actions” are completely different.

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