Thank you your short posting ,as it does answer this question very well.
I once heard a priest explain it this way------The human person is a wonderful example of God’s Creative Design and is in fact the summit of it.God created mankind and He found it good.We read in the scriptures "Thank you Lord,for I am wonderfully made’.
God created man & a woman and He commanded them to increase and multiply.So a married couple are free to enjoy each other sexually and God can bless this union which might beget a child and He breaths life into that child.
(1)Now using an example— A man might have married the woman of his dreams and on the honeymoon they can enjoy each other sexually .If they offer this pleasure up to God as included in the morning offering prayer(all our prays,works,joys & sufferings etc) they even gain merit for this sexual act—a sign of their love for each other.
(2)But let us say that his best man had lustful thoughts about his buddy;he could be imagining all sorts of arousing fantasies on what they could be doing.Now ,if these were just passing thoughts, then there would be not sin.But if he was aware of them,but still entertained these thoughts deliberately, then he would be committing a sin.
Example one (1) is something good, but the second(2) is sinful.Why? Whilst the activities of a newly married couple are good;his best buddy puts a twisted slant on it,with the purpose of obtaining sexual gratification.
Now this example I believe, can apply to the question asked in the thread.An attractive person have presented themselves and in this case fully clothed.This could be in a movie,photograph or someone walking along the street;all just going about their daily business.A person might be sexually attracted ,which can be an uncontrolled imaginings or desire.This can be just part of our fallen human condition.But if a person uses these persons or images in order to gratify themselves sexually(lust); then again, they have twisted and warped their mind.
I remember a person telling me that when he visited a prison which had a large proportion of sex offending prisoners ,that he was instructed not to have any newspapers/magazines with photographs of young boys or girls. Whilst this is on the extreme edge of our society,never the less ,these people use innocent children as tools for their inordinate sexual urges.
Watching pornography can “re-wire” the brain and it twists and warps a perception of the God given gift of sex and the possibility of partaking in God’s own creative powers.
Oh Most Pure Heart of Mary,make me pure in body and in mind and in heart.Amen.